FREEMEDSITE MRCS ACADEMY • ARTICLE 12 • PART A PAPER 1

Renal, Acid–Base and Electrolyte Physiology Made Easy for MRCS Part A

Understand nephron transport, GFR, ADH and aldosterone, sodium and potassium disorders, arterial blood gases and perioperative acute kidney injury through practical surgical scenarios.

12Teaching modules

52Saved checklist topics

40Explained 5-option SBAs

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Who should use this?

MRCS Part A candidates, surgical trainees and clinicians revising kidney physiology from first principles. Learn the concept, connect it to an on-call decision, then test yourself with original SBAs. Clinical recommendations are educational summaries and should never replace local protocols.

How to use: Read a module → tick each relevant revision objective → answer its SBA questions → revise incorrect answers using the saved quiz.

Contents — choose a learning section

Kidney function & nephronGFR & renal clearanceTubular transportADH & RAASUrine concentrationSodium & osmolalityPotassium & magnesiumABG interpretationAcid–base patternsRenal tubular acidosisFluids & acute kidney injuryHigh-yield summarySaved revision checklist40 clinical SBAsFAQsReferences & next links

1. Kidney function and the nephron

The kidney is not merely a filter. It adjusts extracellular volume, sodium concentration, potassium balance, osmolality and pH while clearing metabolic waste. These processes are essential to a surgical patient with haemorrhage, burns, sepsis, urinary obstruction or postoperative oliguria.

Glomerulus

Filtration of plasma water and small solutes. Protein-rich blood normally remains within the circulation.

Proximal tubule

Bulk reabsorption of sodium, water, filtered glucose, amino acids and much bicarbonate.

Loop of Henle

Descending thin limb passes water; thick ascending limb reabsorbs Na–K–2Cl yet is relatively water-impermeable.

Distal nephron

Fine regulation of sodium, potassium, hydrogen ions and water by aldosterone and ADH.

Clinical application: Two patients can have equally low urine output with entirely different causes: reduced renal perfusion, intrinsic renal injury or urinary outflow obstruction. Always assess the whole patient, medication history, haemodynamics and catheter patency.

2. Filtration, clearance, renal perfusion and calculations

GFR is the rate at which plasma water is filtered through glomerular capillaries. In healthy younger adults, it is roughly 90–120 mL/min/1.73 m², varies with age and body size, and must be interpreted against clinical context. Renal plasma flow is the plasma delivered to the kidneys per minute, not the urine production rate.

Clearance Cx = (Urine concentration of x × Urine flow) / Plasma concentration of x

Filtration fraction = GFR / Renal plasma flow; approximately 0.20 in health

Inulin

Freely filtered with no significant secretion or reabsorption: classical measured-GFR marker.

Creatinine

Convenient endogenous marker; mild tubular secretion means its clearance can overestimate true GFR.

PAH

At low plasma concentrations, filtration plus secretion gives effective renal plasma flow; extraction is not literally complete.

Afferent vs efferent

Afferent constriction generally reduces renal blood flow and GFR. Modest efferent constriction initially preserves/increases filtration fraction but severe efferent constriction can lower GFR.

Glomerular filtration pressure depends on glomerular capillary hydraulic pressure opposed by Bowman's-space hydraulic pressure and capillary oncotic pressure. Urinary obstruction raises pressure in Bowman's space and reduces net filtration. In a volume-depleted patient, NSAIDs reduce prostaglandin-mediated afferent support; ACE inhibitors/ARBs reduce angiotensin-II-mediated efferent tone.

Exam trap: A transient increase in serum creatinine does not automatically imply an identical percentage decrease in GFR, especially before creatinine reaches a new steady state.

3. Segment-by-segment transport and diuretics

Many MRCS questions can be solved by knowing where a solute is transported, which protein transports it and which drug inhibits that protein.

Proximal convoluted tubule

Reabsorbs about two-thirds of filtered sodium and water; SGLT2 cotransports most filtered glucose; carbonic anhydrase facilitates bicarbonate recovery.

Thick ascending limb

NKCC2 takes up Na+, K+ and 2Cl−. Impermeable to water; furosemide inhibits this carrier, impairing medullary hypertonicity.

Early distal convoluted tubule

NCC carries Na+ and Cl−. Thiazides inhibit NCC; this is a major site of regulated calcium reabsorption.

Collecting duct

Principal cells use ENaC to absorb sodium and secrete potassium; intercalated cells participate in acid–base regulation.

Filtered load of a freely filtered solute = GFR × plasma concentration

Aldosterone increases apical ENaC activity and basolateral Na+/K+-ATPase in principal cells, favouring sodium reabsorption with potassium secretion. The lumen-negative potential also promotes distal hydrogen-ion secretion. A loop diuretic can cause volume depletion, potassium loss and metabolic alkalosis; a potassium-sparing drug may cause hyperkalaemia.

4. ADH, RAAS and water balance

ADH (vasopressin) responds strongly to increased plasma osmolality and, in substantial volume depletion, to non-osmotic baroreceptor stimuli. ADH binds V2 receptors in collecting-duct principal cells, causing insertion of aquaporin-2 water channels into the apical membrane.

Renin

Juxtaglomerular cells release renin with reduced renal perfusion, beta-1 sympathetic input or reduced macula-densa NaCl delivery.

Angiotensin II

Constricts arterioles (including efferent tone at usual physiological concentrations), promotes thirst and aldosterone secretion.

Aldosterone

Controls distal sodium retention and potassium excretion; principally an electrolyte/volume regulator.

ADH

Controls free-water retention and urine concentration; excessive action tends to lower serum sodium via water excess.

Practical differentiation: Aldosterone saves salt; ADH saves water. In a stressed postoperative patient, pain, nausea, opiates and physiological stress can increase ADH even when plasma is hypotonic.

5. Concentrating urine and countercurrent physiology

The descending limb of the loop of Henle is relatively water-permeable, while the thick ascending limb actively removes NaCl without following water. This countercurrent multiplier helps create a concentrated renal medulla. The vasa recta act as countercurrent exchangers, limiting washout of that medullary gradient.

When ADH is present, collecting ducts become water permeable and water moves into the hyperosmotic medulla, producing smaller volumes of concentrated urine. Urea recycling contributes to inner medullary hypertonicity. Without ADH action, collecting ducts remain poorly permeable to water and large amounts of dilute urine can be passed.

Central diabetes insipidus

ADH secretion is deficient; urine is dilute and responds to desmopressin.

Nephrogenic diabetes insipidus

ADH is present but the kidney responds poorly; little or no urine-concentrating response to desmopressin.

SIADH

ADH action persists despite hypotonicity, producing inappropriately concentrated urine in an appropriate clinical setting.

Loop diuretic

Disrupts the medullary gradient and reduces maximal ability to concentrate urine.

6. Sodium, plasma osmolality and hyponatraemia

The plasma sodium concentration reflects the ratio of exchangeable body sodium plus potassium to total body water. Therefore a patient with hyponatraemia does not necessarily lack total-body sodium: most cases involve relative water excess. Begin by checking whether plasma is hypotonic; consider hyperglycaemia and other causes of non-hypotonic hyponatraemia.

Calculated serum osmolality (mmol/L inputs) ≈ 2 × Na + glucose + urea

An alternative U.S. formula (mg/dL inputs): 2 × Na + glucose/18 + BUN/2.8

For hypotonic hyponatraemia, consider symptom severity, duration, clinical volume status, urine osmolality and urine sodium. SIADH is a diagnosis of exclusion: examine adrenal/thyroid function, drugs, renal function and clinical context. An elevated urine sodium alone does not diagnose SIADH.

Emergency context: Severe symptomatic hyponatraemia (for example seizures) requires prompt monitored hypertonic saline under an established protocol. Do not attempt rapid normalisation of sodium; overcorrection, particularly of chronic hyponatraemia, risks osmotic demyelination.

Hypovolaemic

GI loss, diuretics or third spacing; normal renal conservation may lower urine sodium unless confounded.

Euvolaemic

Consider SIADH, glucocorticoid deficiency, hypothyroidism and excess water intake.

Hypervolaemic

Heart failure, cirrhosis or kidney failure; reduced effective circulating volume can activate ADH.

Hypernatraemia

Usually relative water deficit, reduced access to water, free-water loss or excess sodium; determine cause and assess volume.

7. Potassium, ECG changes and magnesium

Potassium's concentration across the cell membrane helps determine resting membrane potential. Insulin and beta-2 agonists promote intracellular potassium movement; aldosterone promotes renal excretion. Acidosis can contribute to hyperkalaemia in some settings, but the effect varies by type of acidosis and other factors.

Severe hyperkalaemia may produce peaked T waves, PR prolongation, QRS widening, a sine-wave pattern or ventricular arrhythmias, but dangerous hyperkalaemia can occur without classic ECG changes. In an unstable patient with ECG toxicity, intravenous calcium acts on membrane excitability promptly; it does not remove potassium or directly lower serum potassium. Insulin–glucose shifts potassium into cells; ongoing monitoring is essential because hypoglycaemia and rebound hyperkalaemia can occur.

Electrolyte pearl: Refractory hypokalaemia frequently coexists with magnesium deficiency. Magnesium depletion increases renal potassium wasting, so correct magnesium as well as potassium.

Hypokalaemia can cause muscle weakness, ileus and arrhythmia; ECG findings may include flattened T waves, ST depression and U waves. Check potassium, magnesium, kidney function, acid–base status and medicines in postoperative patients.

8. A four-step acid–base and ABG interpretation method

Read the ABG as an integrated pattern, not just an isolated pH. First decide acidaemia or alkalaemia; next identify the primary respiratory (PaCO2) or metabolic (bicarbonate) abnormality; then calculate the expected compensation; finally look for additional disorders and, for metabolic acidosis, the anion gap.

Anion gap (excluding K) = Na − (Cl + HCO3)

Winter formula: expected PaCO2 (mmHg) = 1.5 × HCO3 + 8 ± 2

Albumin correction: corrected AG ≈ measured AG + 2.5 × (4 − albumin in g/dL)

Reference ranges vary by laboratory and method. A common normal AG approximation without potassium is 8–12 mmol/L, but the lab's own reference range and albumin matter. In Winter's formula, measured PaCO2 above the predicted range indicates an additional respiratory acidosis; below the range indicates an additional respiratory alkalosis.

Exam trap: Physiological compensation does not overshoot to create the opposite pH disorder. If PaCO2/HCO3 changes are more or less than expected, consider a mixed disorder.

9. Recognising acid–base patterns quickly

Metabolic acidosis

Low bicarbonate; usually respiratory compensation with lower PaCO2. High AG: lactate, ketones, selected toxins, uraemia; normal AG: diarrhoea and renal tubular acidosis.

Metabolic alkalosis

High bicarbonate; typically vomiting, nasogastric suction, loop/thiazide diuretics or mineralocorticoid effects.

Respiratory acidosis

High PaCO2 due to alveolar hypoventilation; acute compensation is limited; chronic renal bicarbonate retention is larger.

Respiratory alkalosis

Low PaCO2 due to hyperventilation; can arise with pain, anxiety, pulmonary embolism or early sepsis.

In chronic respiratory acidosis, serum bicarbonate increases roughly 3.5–4 mmol/L per 10 mmHg chronic rise in PaCO2; in acute respiratory acidosis it increases only about 1 mmol/L per 10 mmHg. In chronic respiratory alkalosis, bicarbonate falls by roughly 4–5 mmol/L per 10 mmHg decrease in PaCO2, whereas the acute reduction is about 2 mmol/L per 10 mmHg.

Vomiting removes HCl from the stomach, producing hypochloraemic metabolic alkalosis with volume and potassium depletion. Diarrhoea usually causes bicarbonate loss and a normal-anion-gap (hyperchloraemic) metabolic acidosis. Marked tissue hypoperfusion may produce lactic acidosis with an elevated anion gap. These patterns can coexist, so a nearly normal pH does not exclude a serious mixed disorder.

10. Renal tubular acidosis and kidney-related acidosis

Renal tubular acidosis (RTA) is a normal-anion-gap metabolic acidosis due to defective renal acid excretion or bicarbonate handling, usually when impairment of filtration alone does not fully explain acidosis.

Type 1 — distal RTA

Distal H+ secretion impaired. Inappropriately alkaline urine despite systemic acidaemia; hypokalaemia and calcium-phosphate stones are classical.

Type 2 — proximal RTA

Impaired proximal bicarbonate reabsorption. During bicarbonate wasting, urine may be alkaline; once plasma bicarbonate is depleted, distal acidification can lower urine pH.

Type 4 — hypoaldosteronism/resistance

Reduced distal K+ and H+ handling, typically hyperkalaemia with mild normal-anion-gap metabolic acidosis.

Advanced kidney failure

Reduced ammonium production/excretion can cause metabolic acidosis, sometimes elevated anion gap as unmeasured anions accumulate.

Clinical reasoning: If a patient has low bicarbonate and high potassium with diabetes or RAAS-blocking medication, consider type 4 RTA, but also evaluate acute kidney injury and medication effects. Urine pH alone is not sufficient to classify all RTAs.

11. Postoperative fluids, oliguria and acute kidney injury

Assess a surgical patient with low urine output systematically: bleeding and hypotension, sepsis, fluid balance, drugs, urinary catheter blockage and urinary obstruction. Do not prescribe fluid boluses just to treat a number without examining perfusion and fluid responsiveness; excess fluid can harm a patient with cardiac or renal dysfunction.

Common adult AKI triggers: creatinine rise ≥26 µmol/L in 48 h OR ≥50% within 7 days OR urine output <0.5 mL/kg/h for >6 h

For a stable adult needing routine IV maintenance, NICE CG174 suggests as an initial consideration approximately 25–30 mL/kg/day water and about 1 mmol/kg/day each of sodium, potassium and chloride, with glucose provision. This is not an automatic prescription: older/frail adults, renal/cardiac impairment, severe fluid losses and postoperative redistribution require individualisation and reassessment.

Do not confuse: resuscitation fluid for shock, replacement fluid for measurable losses, and daily maintenance requirements. Their composition, rate and goals are different.

Seek urgent senior/nephrology assessment for refractory hyperkalaemia, severe acidosis, pulmonary oedema refractory to medical therapy or uraemic complications. No single creatinine or urea value automatically determines dialysis; the whole clinical condition does.

12. One-page high-yield facts and examiner traps

Filtration vs concentration

GFR depends on glomerular filtration pressures; urine concentration depends on the medullary gradient and ADH-dependent collecting duct permeability.

ADH vs aldosterone

ADH retains water via AQP2; aldosterone retains sodium and facilitates potassium secretion via distal principal cells.

Hyperkalaemia actions

IV calcium protects the myocardium, insulin–glucose redistributes potassium, and kidney-based clearance/dialysis removes potassium.

ABG strategy

Check pH, identify primary disturbance, calculate compensation, then anion gap and additional processes.

Vomiting vs diarrhoea

Vomiting tends toward hypochloraemic alkalosis; diarrhoea often leads to hyperchloraemic metabolic acidosis.

Do not trust one marker

Normal ECG does not exclude serious hyperkalaemia; preserved urine output does not rule out AKI; FeNa is context-dependent.

Clinical safety: Numerical cut-offs in this revision chapter are educational. For real patients, use up-to-date local guidelines and supervised clinical judgement, particularly for sodium correction, potassium emergencies, IV fluids and renal replacement decisions.

Interactive revision checklist — 52 high-yield tasks

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Nephron and filtration

Major nephron regions and functions GFR definition and determinants Inulin clearance Creatinine clearance limitations PAH and effective renal plasma flow Filtration fraction and calculation Afferent vs efferent changes Obstruction and Bowmans space back pressure

Tubular transport

PCT bulk sodium and water reabsorption SGLT2 and glucose transport Loop NKCC2 and furosemide Early DCT NCC and thiazides Distal ENaC and aldosterone Alpha and beta intercalated cells Macula-densa NaCl sensing Diuretic drug-target localisation

Water balance and hormones

ADH V2 receptor and AQP2 Renin release triggers Angiotensin II effects Aldosterone and potassium secretion Countercurrent multiplier Vasa recta countercurrent exchange Central vs nephrogenic DI Urea recycling and inner medulla

Sodium and potassium

Serum osmolality calculation Effective osmolality versus tonicity Hyponatraemia categories SIADH diagnostic exclusions Hypernatraemia as relative water deficit Hyperkalaemia ECG caveats IV calcium versus insulin-glucose Hypokalaemia and magnesium Suppressed ADH and dilute urine

Acid–base interpretation

pH and primary disturbance Anion gap formula Albumin-corrected anion gap Winter formula Mixed acid–base disorders Acute respiratory compensation Chronic respiratory compensation Metabolic acidosis causes Hyperchloraemic acidosis after saline

Surgical applications

Vomiting and hypochloraemic alkalosis Diarrhoea and normal-gap acidosis Urine chloride in alkalosis Distal versus proximal RTA Type 4 RTA and hyperkalaemia AKI diagnostic thresholds Postoperative oliguria evaluation FeNa limitations with diuretics Routine IV maintenance principles Urgent RRT clinical indications

40 original MRCS-style clinical SBAs

Five options (A–E) for every question, with detailed teaching, option-by-option explanations, exam pearls, examiner traps and an answered viva extension. Correct options are balanced: eight each in A, B, C, D and E.

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Question 1 of 40

Question 1 of 40 • GFR measurement

Q01. A physiology study needs a filtration marker that is freely filtered and neither reabsorbed nor secreted. Which clearance most closely measures true GFR?

A. Creatinine clearance B. PAH clearance C. Inulin clearance D. Urea clearance E. Sodium clearance

View detailed teaching explanation and answered viva

Correct answer: C. Inulin clearance.

Why correct: Inulin is freely filtered and is not handled significantly by renal tubules, so the filtered load is reflected in excretion. It is the classic reference marker for measuring GFR.

Why the other options are incorrect:

A. Creatinine clearance: Creatinine is secreted in small amounts, causing modest overestimation of true GFR.

B. PAH clearance: PAH extraction approximates effective renal plasma flow, not GFR.

D. Urea clearance: Urea is reabsorbed variably, so clearance substantially underestimates GFR.

E. Sodium clearance: Filtered sodium is almost entirely reabsorbed, giving no direct measurement of GFR.

Exam pearl: Inulin = GFR; PAH ≈ effective RPF; creatinine slightly overestimates GFR.

Examiner trap: Do not equate creatinine clearance with the ideal filtration marker.

Viva extension — Why is inulin impractical for routine clinical practice?
Answer: It requires exogenous administration, steady-state conditions and accurately timed samples.

Question 2 of 40 • Clearance interpretation

Q02. A patient's measured creatinine clearance is slightly higher than a reference GFR measured by an exogenous filtration marker. What explains the difference?

A. PAH competition at the glomerulus B. Complete protein binding of creatinine C. Inulin secretion D. Significant tubular reabsorption of creatinine E. Tubular secretion of creatinine

View detailed teaching explanation and answered viva

Correct answer: E. Tubular secretion of creatinine.

Why correct: Creatinine is freely filtered and a small quantity is secreted by the proximal tubules. Consequently urinary clearance can be higher than actual glomerular filtration.

Why the other options are incorrect:

A. PAH competition at the glomerulus: PAH clearance does not explain overestimation of GFR by creatinine.

B. Complete protein binding of creatinine: Creatinine is not significantly protein-bound in a way that accounts for this pattern.

C. Inulin secretion: Inulin is not secreted and is the comparator, not the cause.

D. Significant tubular reabsorption of creatinine: Reabsorption is not the reason creatinine clearance is usually high.

Exam pearl: Creatinine is not an ideal filtration tracer because some is secreted.

Examiner trap: Serum creatinine also lags during rapidly evolving AKI.

Viva extension — Why does low muscle mass matter when interpreting serum creatinine?
Answer: It reduces creatinine generation, so substantial renal dysfunction may coexist with a deceptively modest serum creatinine rise.

Question 3 of 40 • Renal plasma flow

Q03. A researcher uses low-dose para-aminohippurate (PAH) clearance to estimate renal blood supply. Which quantity is approximated?

A. Urine formation rate B. Effective renal plasma flow C. Filtration fraction D. Renal blood flow including red cells E. Glomerular filtration rate

View detailed teaching explanation and answered viva

Correct answer: B. Effective renal plasma flow.

Why correct: PAH is filtered and actively secreted, so most PAH arriving in perfusing renal plasma is removed in one pass at low concentrations. Its clearance therefore approximates effective renal plasma flow.

Why the other options are incorrect:

A. Urine formation rate: Urine flow is part of the clearance equation but not the quantity PAH represents.

C. Filtration fraction: Filtration fraction is calculated from GFR divided by renal plasma flow.

D. Renal blood flow including red cells: PAH clearance describes a plasma volume; haematocrit is needed to estimate whole-blood flow.

E. Glomerular filtration rate: A freely filtered, non-secreted marker is needed for GFR.

Exam pearl: RBF ≈ RPF/(1 − haematocrit), with RPF an effective estimate here.

Examiner trap: PAH extraction is not literally 100% and falls when secretion saturates.

Viva extension — How is filtration fraction calculated?
Answer: GFR divided by renal plasma flow; normally approximately 0.2.

Question 4 of 40 • Efferent arteriole

Q04. During moderate hypovolaemia, angiotensin II preferentially increases efferent arteriolar resistance. What is the initial effect on filtration fraction?

A. Filtration fraction rises B. No haemodynamic effect C. Filtration fraction must become zero D. Both renal plasma flow and filtration fraction rise E. Glomerular pressure always falls immediately

View detailed teaching explanation and answered viva

Correct answer: A. Filtration fraction rises.

Why correct: Modest efferent constriction raises glomerular hydrostatic pressure relative to falling renal plasma flow, helping preserve filtration and raising filtration fraction. Severe constriction can eventually reduce GFR.

Why the other options are incorrect:

B. No haemodynamic effect: Angiotensin II changes intrarenal resistance and filtration dynamics.

C. Filtration fraction must become zero: Glomerular filtration remains possible in moderate efferent constriction.

D. Both renal plasma flow and filtration fraction rise: Efferent constriction normally reduces renal plasma flow.

E. Glomerular pressure always falls immediately: Moderate efferent constriction initially raises glomerular capillary pressure.

Exam pearl: Moderate efferent constriction: lower RPF, relatively maintained GFR, higher FF.

Examiner trap: Do not generalise this to extreme vasoconstriction.

Viva extension — Why can ACE inhibition lower GFR in bilateral renal artery stenosis?
Answer: It reduces angiotensin-II-mediated efferent support when renal perfusion pressure is already low.

Question 5 of 40 • Afferent autoregulation

Q05. An adult receives an NSAID during volume depletion. Which change can contribute to acute deterioration in GFR?

A. Increased aquaporin insertion B. Increased inulin secretion C. Primary activation of ENaC D. Loss of prostaglandin-supported afferent vasodilatation E. Complete blockade of glomerular filtration by haematuria

View detailed teaching explanation and answered viva

Correct answer: D. Loss of prostaglandin-supported afferent vasodilatation.

Why correct: In reduced effective circulating volume, prostaglandins help maintain afferent blood flow. NSAID inhibition can favour afferent constriction and reduce glomerular capillary pressure and GFR.

Why the other options are incorrect:

A. Increased aquaporin insertion: ADH-dependent aquaporins change collecting-duct water permeability rather than renal perfusion.

B. Increased inulin secretion: Inulin is not secreted and NSAIDs do not cause this mechanism.

C. Primary activation of ENaC: ENaC changes sodium transport, not the described afferent haemodynamic insult.

E. Complete blockade of glomerular filtration by haematuria: Haematuria is not the typical immediate pharmacological mechanism.

Exam pearl: NSAIDs impair afferent support; ACEi/ARB can reduce efferent tone.

Examiner trap: Volume status determines the clinical impact.

Viva extension — Which perioperative factors heighten this risk?
Answer: Dehydration, sepsis, heart failure, chronic kidney disease and concomitant nephrotoxic medicines.

Question 6 of 40 • Obstruction and GFR

Q06. After pelvic surgery, urinary outflow obstruction produces an acute rise in pressure in the renal tubular system. What directly decreases net filtration pressure?

A. Lower Bowman's-space pressure B. Increased filtration coefficient C. Increased afferent flow alone D. Increased Bowman's-space hydrostatic pressure E. Reduced glomerular oncotic pressure

View detailed teaching explanation and answered viva

Correct answer: D. Increased Bowman's-space hydrostatic pressure.

Why correct: Back pressure from obstruction elevates hydrostatic pressure opposing filtration at Bowman's space, reducing net filtration even if the glomerular capillary pressure has not changed proportionately.

Why the other options are incorrect:

A. Lower Bowman's-space pressure: Lower opposing pressure tends to favour filtration.

B. Increased filtration coefficient: A higher filtration coefficient generally favours filtration.

C. Increased afferent flow alone: That does not directly represent the obstructive back-pressure mechanism.

E. Reduced glomerular oncotic pressure: Lower capillary oncotic pressure would favour filtration, not explain the decline.

Exam pearl: Obstruction raises downstream hydraulic pressure and lowers net filtration.

Examiner trap: Do not label all postoperative oliguria as dehydration.

Viva extension — What simple bedside check should occur first with a catheterised patient?
Answer: Check for kinks, blockage and correct catheter placement, alongside assessing the patient.

Question 7 of 40 • Proximal sodium transport

Q07. In a normal kidney, which nephron segment reabsorbs the greatest proportion of filtered sodium and water?

A. Cortical collecting duct B. Proximal convoluted tubule C. Thin ascending limb D. Thick ascending limb E. Early distal convoluted tubule

View detailed teaching explanation and answered viva

Correct answer: B. Proximal convoluted tubule.

Why correct: The proximal tubule reabsorbs approximately two-thirds of filtered sodium and water through largely iso-osmotic transport. Other segments fine-tune the remaining amount.

Why the other options are incorrect:

A. Cortical collecting duct: It regulates final sodium excretion rather than most bulk reabsorption.

C. Thin ascending limb: It is not the principal site of bulk water and sodium reabsorption.

D. Thick ascending limb: It reabsorbs a significant smaller fraction of sodium and is relatively impermeable to water.

E. Early distal convoluted tubule: Only a smaller fraction of filtered sodium reaches and is reabsorbed here.

Exam pearl: Proximal tubule is the principal bulk-reabsorption site.

Examiner trap: Do not confuse a segment with strong regulation with the greatest absolute reabsorption.

Viva extension — How does proximal water movement differ from thick ascending limb?
Answer: Proximal transport is approximately iso-osmotic; thick ascending limb reabsorbs salt without water.

Question 8 of 40 • Glucose handling

Q08. An SGLT2 inhibitor increases glucosuria in a patient with diabetes. Where is the main inhibited transport process located?

A. Early proximal tubule B. Glomerular capillary wall C. Thick ascending limb D. Collecting duct principal cell E. Macula densa

View detailed teaching explanation and answered viva

Correct answer: A. Early proximal tubule.

Why correct: SGLT2 on the apical surface of early proximal tubular cells reabsorbs most filtered glucose with sodium. Inhibiting it increases urinary glucose excretion.

Why the other options are incorrect:

B. Glomerular capillary wall: Glucose filtration occurs there, but active reclamation requires tubular transport.

C. Thick ascending limb: NKCC2 rather than SGLT2 predominates in sodium transport there.

D. Collecting duct principal cell: ENaC and aquaporins are characteristic transport systems; glucose recovery is upstream.

E. Macula densa: This senses tubular NaCl delivery; it is not the bulk glucose-reabsorption site.

Exam pearl: SGLT2: early proximal nephron; SGLT1 handles a smaller remaining fraction downstream.

Examiner trap: Glucosuria can occur due to a drug even without uncontrolled hyperglycaemia.

Viva extension — What transport relationship links SGLT2 and sodium?
Answer: Glucose is cotransported with sodium across the luminal membrane.

Question 9 of 40 • Loop transport

Q09. A patient receives IV furosemide for acute pulmonary congestion. Which transporter is blocked?

A. Na+–Cl− cotransporter (NCC) B. ENaC C. Na+–K+–2Cl− cotransporter (NKCC2) D. SGLT2 E. Aquaporin-2

View detailed teaching explanation and answered viva

Correct answer: C. Na+–K+–2Cl− cotransporter (NKCC2).

Why correct: Loop diuretics inhibit NKCC2 in the thick ascending limb, increasing salt excretion and impairing the corticomedullary concentration gradient.

Why the other options are incorrect:

A. Na+–Cl− cotransporter (NCC): NCC is the target of thiazides in the early distal convoluted tubule.

B. ENaC: Amiloride targets ENaC in distal principal cells.

D. SGLT2: SGLT2 inhibitors act in the proximal tubule.

E. Aquaporin-2: Aquaporin-2 is regulated by ADH in collecting ducts, not inhibited by furosemide directly.

Exam pearl: Furosemide = NKCC2, thick ascending limb, water-impermeable segment.

Examiner trap: Diuretic names are easier to remember when mapped to the nephron.

Viva extension — Why can loop diuretics impair urine concentration?
Answer: They reduce medullary salt accumulation and therefore diminish the osmotic gradient for water extraction.

Question 10 of 40 • Thiazide target

Q10. A patient starts hydrochlorothiazide for hypertension. At which tubular transporter is its principal natriuretic action?

A. SGLT2 in proximal tubule B. ENaC in collecting duct C. V2 receptor in collecting duct D. NKCC2 in the thick ascending limb E. Na+–Cl− cotransporter in early DCT

View detailed teaching explanation and answered viva

Correct answer: E. Na+–Cl− cotransporter in early DCT.

Why correct: Thiazide diuretics inhibit NCC in the early distal convoluted tubule, reducing NaCl reabsorption. They can promote calcium retention and lead to hyponatraemia in susceptible patients.

Why the other options are incorrect:

A. SGLT2 in proximal tubule: This is the glucose-cotransport target of SGLT2 inhibitors.

B. ENaC in collecting duct: Amiloride and triamterene inhibit ENaC.

C. V2 receptor in collecting duct: Vaptans interfere with vasopressin action rather than NCC.

D. NKCC2 in the thick ascending limb: This is inhibited by loop diuretics, not thiazides.

Exam pearl: Thiazide = NCC in DCT; loop = NKCC2 in thick ascending limb.

Examiner trap: Thiazides can produce hyponatraemia despite relatively mild diuresis.

Viva extension — How do thiazides tend to affect urinary calcium?
Answer: They increase renal calcium reabsorption, lowering urinary calcium in many patients.

Question 11 of 40 • ADH mechanism

Q11. After major surgery, pain and nausea stimulate ADH release. Which action most directly reduces renal free-water excretion?

A. Suppression of ENaC by ADH B. Inhibition of renal prostaglandins C. Direct upregulation of SGLT2 D. Blockade of NKCC2 E. V2 receptor–mediated insertion of aquaporin-2 in collecting ducts

View detailed teaching explanation and answered viva

Correct answer: E. V2 receptor–mediated insertion of aquaporin-2 in collecting ducts.

Why correct: ADH activates V2 receptors, increasing cAMP signalling and AQP2 insertion into the apical membrane of principal cells; water follows the medullary osmotic gradient.

Why the other options are incorrect:

A. Suppression of ENaC by ADH: Distal sodium channels are not the principal mediator of water retention.

B. Inhibition of renal prostaglandins: This does not describe the specific V2/AQP2 mechanism.

C. Direct upregulation of SGLT2: SGLT2 reclaims glucose in the proximal tubule and is not the primary ADH action.

D. Blockade of NKCC2: NKCC2 blockade impairs concentration rather than producing ADH water retention.

Exam pearl: ADH controls water permeability via V2 and AQP2.

Examiner trap: Do not confuse aldosterone-driven sodium retention with ADH-driven free-water retention.

Viva extension — Where is AQP2 stored before ADH acts?
Answer: In intracellular vesicles that fuse with the luminal plasma membrane in response to ADH signalling.

Question 12 of 40 • Aldosterone physiology

Q12. A patient with hyperaldosteronism develops hypertension, hypokalaemia and mild alkalosis. Which distal tubular effect is primary?

A. Increased ENaC-mediated sodium absorption and potassium secretion B. NKCC2 blockade C. Inhibition of basolateral Na+/K+-ATPase D. Direct inhibition of AQP2 insertion E. Loss of proximal glucose transport

View detailed teaching explanation and answered viva

Correct answer: A. Increased ENaC-mediated sodium absorption and potassium secretion.

Why correct: Aldosterone stimulates ENaC and basolateral Na+/K+-ATPase in principal cells, promoting electrogenic sodium reabsorption and potassium loss; distal hydrogen handling can favour alkalosis.

Why the other options are incorrect:

B. NKCC2 blockade: This is a loop-diuretic mechanism rather than aldosterone action.

C. Inhibition of basolateral Na+/K+-ATPase: Aldosterone enhances rather than inhibits its activity.

D. Direct inhibition of AQP2 insertion: AQP2 is chiefly regulated by ADH.

E. Loss of proximal glucose transport: Glucose cotransport is not the dominant aldosterone target.

Exam pearl: Aldosterone retains sodium and facilitates secretion of potassium and hydrogen ions.

Examiner trap: Do not treat its effects as identical to ADH.

Viva extension — How can hyperaldosteronism contribute to metabolic alkalosis?
Answer: Enhanced distal sodium reabsorption creates conditions favouring hydrogen-ion secretion, with associated potassium depletion.

Question 13 of 40 • Intercalated cells

Q13. An acidotic patient requires increased distal urinary acid secretion. Which cell class is principally adapted to secrete H+ into collecting-duct urine?

A. Principal cells B. Podocytes C. Alpha-intercalated cells D. Juxtaglomerular cells E. Macula densa cells

View detailed teaching explanation and answered viva

Correct answer: C. Alpha-intercalated cells.

Why correct: Alpha-intercalated cells secrete hydrogen ions via apical proton pumps and return bicarbonate to blood. Beta-intercalated cells can secrete bicarbonate instead.

Why the other options are incorrect:

A. Principal cells: They primarily handle sodium, potassium and ADH-regulated water transport.

B. Podocytes: Podocytes form a filtration-barrier layer, not collecting-duct acid secretion.

D. Juxtaglomerular cells: They synthesise renin rather than carry out distal luminal acid excretion.

E. Macula densa cells: They sense distal NaCl delivery and mediate tubuloglomerular feedback.

Exam pearl: Alpha cells secrete acid; beta cells can secrete bicarbonate.

Examiner trap: Distal acidification is impaired in type 1 RTA.

Viva extension — Which additional process makes renal acid excretion effective?
Answer: Excretion of hydrogen with urinary buffers such as ammonium and titratable phosphate.

Question 14 of 40 • Countercurrent exchange

Q14. Which vascular arrangement preserves the renal medullary osmotic gradient during urine concentration?

A. Only afferent arteriolar constriction B. Bicarbonate secretion in the proximal tubule C. Collecting-duct potassium secretion alone D. Hairpin vasa recta acting as countercurrent exchangers E. High-flow straight capillaries washing the medulla

View detailed teaching explanation and answered viva

Correct answer: D. Hairpin vasa recta acting as countercurrent exchangers.

Why correct: The medullary capillaries exchange water and solutes along their descending and ascending limbs. Their slow flow and hairpin arrangement minimise loss of the gradient.

Why the other options are incorrect:

A. Only afferent arteriolar constriction: It changes filtration haemodynamics, not the exchange architecture.

B. Bicarbonate secretion in the proximal tubule: This is unrelated to medullary gradient preservation.

C. Collecting-duct potassium secretion alone: It cannot create the countercurrent exchange pattern.

E. High-flow straight capillaries washing the medulla: High medullary blood flow would tend to wash out interstitial solutes.

Exam pearl: Loop multiplies a gradient; vasa recta preserve it.

Examiner trap: Countercurrent multiplier and exchanger are not synonyms.

Viva extension — What physical property of descending thin limb supports concentration?
Answer: High permeability to water allows water movement into hypertonic medullary interstitium.

Question 15 of 40 • Medullary gradient

Q15. A loop diuretic markedly reduces maximal urine concentration even when ADH is present. Why?

A. It stimulates maximal bicarbonate secretion B. It reduces medullary NaCl accumulation C. It abolishes all renal perfusion D. It directly destroys all aquaporin-2 proteins E. It blocks urea filtration at the glomerulus

View detailed teaching explanation and answered viva

Correct answer: B. It reduces medullary NaCl accumulation.

Why correct: Inhibiting NKCC2 reduces salt delivery to the medullary interstitium, weakening the hyperosmotic gradient required to pull water out of collecting ducts.

Why the other options are incorrect:

A. It stimulates maximal bicarbonate secretion: This does not explain the impaired concentrating gradient.

C. It abolishes all renal perfusion: Loop diuretics do not normally stop blood flow to the kidneys.

D. It directly destroys all aquaporin-2 proteins: AQP2 can remain ADH-responsive while the osmotic gradient is diminished.

E. It blocks urea filtration at the glomerulus: Urea filtration is not the loop diuretic target.

Exam pearl: Gradient AND water permeability are both required for concentrated urine.

Examiner trap: ADH alone cannot produce maximal concentration without an intact medullary gradient.

Viva extension — Which nephron segment establishes much of the medullary salt gradient?
Answer: The thick ascending limb via active NKCC2-mediated NaCl transport.

Question 16 of 40 • Tubuloglomerular feedback

Q16. The macula densa senses a fall in tubular NaCl delivery. Which signal is most closely linked to initiation of renin release?

A. Complete cessation of afferent blood flow B. Reduced NaCl sensed through macula-densa NKCC2 C. Rising distal NaCl is always the renin trigger D. Increased urine glucose alone E. High plasma calcium directly

View detailed teaching explanation and answered viva

Correct answer: B. Reduced NaCl sensed through macula-densa NKCC2.

Why correct: Macula-densa cells sense luminal NaCl and signal juxtaglomerular renin secretion when delivery is low; sympathetic beta-1 input and low renal perfusion also stimulate renin.

Why the other options are incorrect:

A. Complete cessation of afferent blood flow: Renin release does not require absence of blood flow.

C. Rising distal NaCl is always the renin trigger: Higher NaCl delivery typically suppresses renin release and can trigger afferent feedback.

D. Increased urine glucose alone: Glucose is not the principal macula-densa sensing variable.

E. High plasma calcium directly: Calcium is not the primary tubular cue for this feedback pathway.

Exam pearl: Macula densa senses NaCl, not serum sodium in isolation.

Examiner trap: Tubular delivery depends on upstream transport and filtration.

Viva extension — What are the other two classical triggers for renin?
Answer: Reduced renal afferent perfusion pressure and renal sympathetic beta-1 stimulation.

Question 17 of 40 • FeNa interpretation

Q17. A septic patient taking furosemide develops AKI. A fractional excretion of sodium (FeNa) is above 2%. What is the best interpretation?

A. This proves intrinsic tubular necrosis B. This excludes low effective circulating volume C. This proves bilateral obstruction D. FeNa is unreliable for distinguishing causes of AKI while diuretics are active E. This excludes sepsis-associated injury

View detailed teaching explanation and answered viva

Correct answer: D. FeNa is unreliable for distinguishing causes of AKI while diuretics are active.

Why correct: Loop diuretics increase urinary sodium excretion irrespective of the underlying haemodynamic or tubular state. Sepsis also produces variable tubular responses, so use clinical context and other tests.

Why the other options are incorrect:

A. This proves intrinsic tubular necrosis: High FeNa is not specific in a diuretic-treated patient.

B. This excludes low effective circulating volume: Volume depletion can coexist with diuretic-induced sodium loss.

C. This proves bilateral obstruction: FeNa has no such diagnostic specificity.

E. This excludes sepsis-associated injury: FeNa does not exclude sepsis or mixed forms of AKI.

Exam pearl: FeNa is a contextual clue, not a diagnosis; diuretics are a major confounder.

Examiner trap: A single fractional excretion value is not a substitute for examining the patient.

Viva extension — What bedside checks can be more immediately helpful?
Answer: Volume assessment, trend in creatinine and urine output, medication review, urine examination and obstruction checks.

Question 18 of 40 • Calculated osmolality

Q18. A postoperative patient has Na 140 mmol/L, glucose 5 mmol/L and urea 5 mmol/L. Approximate calculated serum osmolality is:

A. 280 mOsm/kg B. 300 mOsm/kg C. 290 mOsm/kg D. 560 mOsm/kg E. 145 mOsm/kg

View detailed teaching explanation and answered viva

Correct answer: C. 290 mOsm/kg.

Why correct: In SI units, calculated osmolarity is approximately 2 × sodium + glucose + urea = 280 + 5 + 5 = 290. It is an approximation that differs from directly measured serum osmolality.

Why the other options are incorrect:

A. 280 mOsm/kg: This omits glucose and urea.

B. 300 mOsm/kg: This incorrectly adds extra solute not present in the supplied values.

D. 560 mOsm/kg: Doubling sodium twice overestimates serum particle concentration.

E. 145 mOsm/kg: This fails to account for sodium-associated counterions.

Exam pearl: Use mmol/L formula for SI lab values; glucose/18 and BUN/2.8 apply to mg/dL.

Examiner trap: Units must be checked before calculating.

Viva extension — How do osmolarity and tonicity differ?
Answer: Tonicity reflects effective osmoles across cell membranes; urea contributes to measured osmolality but is usually not an effective sustained osmole.

Question 19 of 40 • Hypernatraemia physiology

Q19. An older patient develops hypernatraemia during impaired oral intake after surgery. What fundamental abnormality usually drives the raised serum sodium?

A. It proves normal volume status B. It proves only renal sodium retention C. It necessarily means total-body sodium overload D. It establishes hyperaldosteronism E. Water deficit relative to exchangeable sodium and potassium

View detailed teaching explanation and answered viva

Correct answer: E. Water deficit relative to exchangeable sodium and potassium.

Why correct: Hypernatraemia most often reflects inadequate water intake or excess water loss compared with total body solute; absolute total-body sodium can be low, normal or high.

Why the other options are incorrect:

A. It proves normal volume status: Volume depletion or occasionally overload may coexist.

B. It proves only renal sodium retention: Extrarenal water loss or limited access to drinking water are common alternatives.

C. It necessarily means total-body sodium overload: Most hypernatraemic patients do not have absolute sodium overload.

D. It establishes hyperaldosteronism: Aldosterone is not required for hypernatraemia.

Exam pearl: Serum sodium concentration is primarily about water-to-solute ratio.

Examiner trap: Do not infer total-body sodium excess from a high concentration.

Viva extension — Which additional assessments are essential before selecting fluid therapy?
Answer: Circulatory status, likely duration, ongoing losses, neurological state and serial electrolytes.

Question 20 of 40 • SIADH physiology

Q20. Following thoracic surgery, Na falls to 123 mmol/L and measured plasma osmolality is low. The patient appears euvolaemic; urine remains concentrated without diuretics. Which mechanism best explains the water retention?

A. Persistent ADH action despite hypotonic plasma B. Nephrogenic ADH resistance C. Complete absence of ADH secretion D. Loss of all renal sodium reabsorption E. Primary osmotic diuresis

View detailed teaching explanation and answered viva

Correct answer: A. Persistent ADH action despite hypotonic plasma.

Why correct: In SIADH, collecting-duct water permeability remains inappropriately high despite hypo-osmolality, causing retention of electrolyte-free water and lowering serum sodium.

Why the other options are incorrect:

B. Nephrogenic ADH resistance: Resistance would reduce urine concentration rather than maintain it.

C. Complete absence of ADH secretion: This would favour polyuria and hypernatraemia, not water retention.

D. Loss of all renal sodium reabsorption: SIADH is principally excess water retention, not a global failure of sodium reclamation.

E. Primary osmotic diuresis: Osmotic diuresis generally increases solute and water loss.

Exam pearl: SIADH is defined by inappropriate antidiuresis plus appropriate exclusions.

Examiner trap: Pain, nausea and drugs can raise ADH postoperatively; consider alternative diagnoses.

Viva extension — What biochemical tests help interpret possible SIADH?
Answer: Serum osmolality, urine osmolality, urine sodium, thyroid/adrenal evaluation and renal function in context.

Question 21 of 40 • Central DI

Q21. After pituitary surgery, a patient passes 6 L/day of dilute urine and becomes hypernatraemic. Urine osmolality rises markedly after desmopressin. Which diagnosis is most likely?

A. Central diabetes insipidus B. Nephrogenic diabetes insipidus C. SIADH D. Acute glomerulonephritis E. Primary aldosteronism

View detailed teaching explanation and answered viva

Correct answer: A. Central diabetes insipidus.

Why correct: Deficient endogenous ADH causes hypotonic polyuria. Desmopressin replaces the missing V2 agonist signal and increases collecting-duct water permeability if renal responsiveness is intact.

Why the other options are incorrect:

B. Nephrogenic diabetes insipidus: The kidney shows little or no rise in urine osmolality after desmopressin in classic complete nephrogenic DI.

C. SIADH: SIADH causes concentrated urine and water retention rather than hypotonic polyuria.

D. Acute glomerulonephritis: This generally does not create a strong desmopressin concentrating response.

E. Primary aldosteronism: Sodium/potassium changes are possible, but the desmopressin response points to ADH deficiency.

Exam pearl: Central DI responds to desmopressin; nephrogenic DI responds poorly.

Examiner trap: Post-pituitary polyuria can have other causes; establish hypotonicity and monitor sodium.

Viva extension — How does desmopressin affect the collecting duct?
Answer: It activates V2 receptors and drives AQP2 insertion.

Question 22 of 40 • Hyperkalaemia and ECG toxicity

Q22. A patient with severe AKI has K 7.1 mmol/L, wide QRS complexes and weakness. What is the immediate membrane-stabilising treatment?

A. Dextrose alone B. Furosemide alone C. Intravenous calcium according to the emergency protocol D. Sodium bicarbonate as the sole therapy E. Oral potassium supplementation

View detailed teaching explanation and answered viva

Correct answer: C. Intravenous calcium according to the emergency protocol.

Why correct: IV calcium stabilises cardiac membrane excitability when dangerous ECG changes are present, acting faster on electrical toxicity than potassium-removal strategies. It does not itself lower the serum potassium.

Why the other options are incorrect:

A. Dextrose alone: Without insulin, dextrose does not provide reliable rapid potassium redistribution.

B. Furosemide alone: Its potassium-lowering action depends on urine output and is not immediate membrane stabilisation.

D. Sodium bicarbonate as the sole therapy: Bicarbonate has limited, context-dependent potassium effects and is not the primary membrane stabiliser.

E. Oral potassium supplementation: Additional potassium can worsen life-threatening hyperkalaemia.

Exam pearl: With ECG toxicity, calcium treats electrical risk; treat potassium concentration in parallel.

Examiner trap: A normal ECG does not rule out severe hyperkalaemia.

Viva extension — What is the next physiological goal after stabilising the heart?
Answer: Temporarily shift potassium into cells and arrange effective potassium removal, while monitoring ECG, potassium and glucose.

Question 23 of 40 • Potassium redistribution

Q23. Insulin with glucose is given for severe hyperkalaemia. Which statement best describes its effect?

A. It permanently cures renal potassium retention B. It reduces urinary potassium excretion to zero C. It directly removes potassium through faeces D. It stabilises the myocardium like IV calcium E. It temporarily shifts K+ into cells without removing total-body potassium

View detailed teaching explanation and answered viva

Correct answer: E. It temporarily shifts K+ into cells without removing total-body potassium.

Why correct: Insulin stimulates cellular sodium-potassium ATPase activity, moving potassium from extracellular fluid into cells. The effect is temporary, so elimination and monitoring remain important.

Why the other options are incorrect:

A. It permanently cures renal potassium retention: Renal excretory failure can cause recurrent hyperkalaemia after the shift wears off.

B. It reduces urinary potassium excretion to zero: The principal acute action is redistribution, not blockade of excretion.

C. It directly removes potassium through faeces: Insulin does not directly bind or eliminate potassium in the gut.

D. It stabilises the myocardium like IV calcium: Calcium has the direct membrane-stabilising role.

Exam pearl: Shift versus eliminate: a crucial three-step emergency distinction.

Examiner trap: Insulin–glucose treatment can cause delayed hypoglycaemia.

Viva extension — Which other agonist can shift potassium into cells?
Answer: Inhaled beta-2 agonists such as salbutamol can assist, with variable response.

Question 24 of 40 • Magnesium with hypokalaemia

Q24. Despite repeated potassium replacement, a postoperative patient remains hypokalaemic and has low serum magnesium. What is the key corrective action?

A. Ignore magnesium because only potassium affects cardiac rhythm B. Stop all potassium replacement permanently C. Give IV calcium as the potassium treatment D. Replace magnesium alongside potassium E. Prescribe an unrestricted loop diuretic

View detailed teaching explanation and answered viva

Correct answer: D. Replace magnesium alongside potassium.

Why correct: Magnesium deficiency promotes ongoing renal potassium loss, partly through changes in distal potassium channel regulation. Persistent hypomagnesaemia makes potassium repletion difficult.

Why the other options are incorrect:

A. Ignore magnesium because only potassium affects cardiac rhythm: Magnesium deficiency also increases arrhythmia risk and impedes potassium correction.

B. Stop all potassium replacement permanently: Potassium may still be required; underlying magnesium loss should be addressed.

C. Give IV calcium as the potassium treatment: Calcium is not the primary therapy for hypokalaemia.

E. Prescribe an unrestricted loop diuretic: Loop diuretics can worsen potassium and magnesium wasting.

Exam pearl: Refractory hypokalaemia: always check magnesium.

Examiner trap: Failure to correct coexisting deficits causes recurrent losses.

Viva extension — Which other clinical features may occur with severe hypomagnesaemia?
Answer: Neuromuscular irritability and increased arrhythmic susceptibility.

Question 25 of 40 • Vomiting-related alkalosis

Q25. A patient has 4 days of persistent vomiting and NG suction. ABG shows pH 7.52 and HCO3 36 mmol/L; chloride is low. Which primary disorder is expected?

A. Normal-anion-gap metabolic acidosis B. Hypochloraemic metabolic alkalosis C. Primary respiratory acidosis D. High-anion-gap metabolic acidosis from vomiting alone E. Primary respiratory alkalosis

View detailed teaching explanation and answered viva

Correct answer: B. Hypochloraemic metabolic alkalosis.

Why correct: Loss of gastric hydrochloric acid increases extracellular bicarbonate relative to hydrogen ions. Chloride and volume depletion then maintain the alkalosis through renal handling.

Why the other options are incorrect:

A. Normal-anion-gap metabolic acidosis: This is typical of loss of intestinal bicarbonate in diarrhoea.

C. Primary respiratory acidosis: That would have elevated PaCO2 as the main driver of acidemia.

D. High-anion-gap metabolic acidosis from vomiting alone: Vomiting alone primarily removes gastric acid; other concomitant illnesses might add a gap acidosis.

E. Primary respiratory alkalosis: High bicarbonate is the primary metabolic change in the presented pattern.

Exam pearl: Upper GI acid loss → metabolic alkalosis; intestinal bicarbonate loss → metabolic acidosis.

Examiner trap: Evaluate volume, chloride and potassium, not bicarbonate alone.

Viva extension — Why can isotonic chloride-containing fluid correct some cases?
Answer: It replaces volume and chloride, allowing renal bicarbonate excretion when the cause is saline-responsive.

Question 26 of 40 • Diarrhoea-related acidosis

Q26. A patient with severe diarrhoea has Na 140, chloride 113 and bicarbonate 16 mmol/L. Which acid–base pattern is most likely?

A. High-anion-gap acidosis by definition B. Pure respiratory alkalosis C. Complete compensation of respiratory acidosis D. Normal-anion-gap hyperchloraemic metabolic acidosis E. Metabolic alkalosis

View detailed teaching explanation and answered viva

Correct answer: D. Normal-anion-gap hyperchloraemic metabolic acidosis.

Why correct: Loss of intestinal bicarbonate causes metabolic acidosis, while chloride rises relative to bicarbonate, often keeping the anion gap in the normal range. Here AG = 140 − (113+16) = 11.

Why the other options are incorrect:

A. High-anion-gap acidosis by definition: The calculated AG is about 11 without potassium, not elevated by typical ranges.

B. Pure respiratory alkalosis: The abnormal bicarbonate and chloride pattern identifies a metabolic process.

C. Complete compensation of respiratory acidosis: Compensation does not explain the specific GI bicarbonate loss history.

E. Metabolic alkalosis: Intestinal bicarbonate loss lowers, rather than raises, serum bicarbonate.

Exam pearl: Diarrhoea often produces normal-gap acidosis; vomiting often produces alkalosis.

Examiner trap: Do not confuse the two GI fluid losses.

Viva extension — Name another cause of normal-anion-gap metabolic acidosis.
Answer: Renal tubular acidosis or large chloride-rich fluid loads.

Question 27 of 40 • Anion gap calculation

Q27. A shocked patient has Na 140 mmol/L, chloride 100 mmol/L and bicarbonate 12 mmol/L. What is the anion gap excluding potassium?

A. 40 mmol/L B. 152 mmol/L C. 12 mmol/L D. 16 mmol/L E. 28 mmol/L

View detailed teaching explanation and answered viva

Correct answer: E. 28 mmol/L.

Why correct: Anion gap = Na − (Cl + HCO3) = 140 − (100 + 12) = 28 mmol/L. This is elevated for many laboratory reference intervals and supports an additional unmeasured-anion process.

Why the other options are incorrect:

A. 40 mmol/L: This incorrectly omits bicarbonate from the subtracted anions.

B. 152 mmol/L: This adds bicarbonate instead of subtracting chloride and bicarbonate.

C. 12 mmol/L: This is the bicarbonate value, not the calculated gap.

D. 16 mmol/L: This may result from incorrect subtraction of a component.

Exam pearl: AG calculation must use the actual measured Na, Cl and bicarbonate with consistent units.

Examiner trap: A high gap identifies unmeasured anions, not their specific cause.

Viva extension — Which high-gap processes are common in surgical practice?
Answer: Lactic acidosis in shock, ketoacidosis and uraemic retention of unmeasured anions.

Question 28 of 40 • Albumin-corrected anion gap

Q28. A critically ill patient has albumin 2.0 g/dL and a measured anion gap of 13 mmol/L. Approximate albumin-corrected anion gap is:

A. 32 mmol/L B. 18 mmol/L C. 8 mmol/L D. 13 mmol/L E. 25 mmol/L

View detailed teaching explanation and answered viva

Correct answer: B. 18 mmol/L.

Why correct: Use the common approximation: corrected AG = measured AG + 2.5 × (4 − albumin in g/dL). Here 13 + 2.5 × 2 = 18 mmol/L.

Why the other options are incorrect:

A. 32 mmol/L: This is not produced by the standard correction.

C. 8 mmol/L: This subtracts the correction rather than adding it.

D. 13 mmol/L: This ignores the masking effect of hypoalbuminaemia.

E. 25 mmol/L: This overcorrects for albumin 2 g/dL.

Exam pearl: Hypoalbuminaemia lowers the baseline uncorrected anion gap.

Examiner trap: Severe illness can hide a high-gap process unless albumin is considered.

Viva extension — Why is albumin so influential?
Answer: Albumin is a major unmeasured negatively charged protein in plasma.

Question 29 of 40 • Winter compensation

Q29. A patient in ketoacidosis has bicarbonate 12 mmol/L and measured PaCO2 24 mmHg. What is Winter formula expected PaCO2?

A. About 12 mmHg B. About 40 mmHg C. About 26 mmHg (acceptable 24–28) D. About 52 mmHg E. About 8 mmHg

View detailed teaching explanation and answered viva

Correct answer: C. About 26 mmHg (acceptable 24–28).

Why correct: Winter formula gives 1.5 × 12 + 8 = 26 mmHg, with approximate range 24–28. The measured 24 is at the lower expected boundary and can reflect appropriate respiratory compensation.

Why the other options are incorrect:

A. About 12 mmHg: This is bicarbonate concentration, not predicted carbon dioxide.

B. About 40 mmHg: This ignores compensatory hyperventilation in metabolic acidosis.

D. About 52 mmHg: This indicates hypoventilation and would worsen the acidemia.

E. About 8 mmHg: This is the fixed additive constant in the formula, not the result.

Exam pearl: Winter: expected PaCO2 = 1.5 × HCO3 + 8 ± 2.

Examiner trap: If observed PaCO2 lies outside the range, look for a second respiratory disturbance.

Viva extension — What if the measured PaCO2 were 40 mmHg in this case?
Answer: Additional respiratory acidosis would be present because respiratory compensation is insufficient.

Question 30 of 40 • Mixed acid-base disorders

Q30. A patient has pH 7.10, HCO3 12 mmol/L and PaCO2 40 mmHg. Which interpretation is most accurate?

A. Metabolic acidosis with superimposed respiratory acidosis B. Pure chronic respiratory alkalosis C. Pure metabolic alkalosis D. Normal physiological compensation without any primary disorder E. Appropriately compensated metabolic acidosis

View detailed teaching explanation and answered viva

Correct answer: A. Metabolic acidosis with superimposed respiratory acidosis.

Why correct: The low bicarbonate causes metabolic acidosis. Winter formula predicts PaCO2 around 26 ± 2, but the measured value of 40 is much higher, indicating an additional failure of ventilation.

Why the other options are incorrect:

B. Pure chronic respiratory alkalosis: PaCO2 is not low and bicarbonate reduction cannot be explained as compensation here.

C. Pure metabolic alkalosis: Bicarbonate is low and the pH indicates acidemia.

D. Normal physiological compensation without any primary disorder: Both pH and bicarbonate are substantially abnormal.

E. Appropriately compensated metabolic acidosis: PaCO2 is not appropriately low for HCO3 of 12.

Exam pearl: Always calculate compensation instead of eyeballing two abnormal values.

Examiner trap: A normal PaCO2 of 40 can be abnormal when severe metabolic acidosis is present.

Viva extension — What clinical scenarios cause this mixture?
Answer: Severe sepsis or shock with concurrent respiratory failure, sedation or exhaustion.

Question 31 of 40 • Acute respiratory alkalosis

Q31. A tachypnoeic patient with suspected pulmonary embolism has pH 7.50, PaCO2 28 mmHg and bicarbonate 22 mmol/L. What is the primary disturbance?

A. High-anion-gap metabolic acidosis B. Acute respiratory alkalosis C. Pure metabolic acidosis with expected compensation D. Primary metabolic alkalosis E. Chronic respiratory acidosis

View detailed teaching explanation and answered viva

Correct answer: B. Acute respiratory alkalosis.

Why correct: The increased pH and reduced PaCO2 indicate a primary respiratory process. Bicarbonate is modestly lower, consistent with acute buffering rather than a primary metabolic acidosis.

Why the other options are incorrect:

A. High-anion-gap metabolic acidosis: No metabolic evidence is given for this, and the pH is alkaline.

C. Pure metabolic acidosis with expected compensation: A low bicarbonate would be the primary abnormality, but here it is only modestly reduced.

D. Primary metabolic alkalosis: The bicarbonate is not elevated; low PaCO2 is driving the alkalaemia.

E. Chronic respiratory acidosis: The PaCO2 is low, not persistently elevated.

Exam pearl: Acute fall in PaCO2 raises pH; causes include pain, anxiety, PE and early sepsis.

Examiner trap: An ABG does not diagnose PE by itself.

Viva extension — How does the kidney adapt if respiratory alkalosis persists for days?
Answer: It reduces bicarbonate reabsorption, producing a larger fall in serum bicarbonate.

Question 32 of 40 • Urine chloride and alkalosis

Q32. An adult has vomiting-related metabolic alkalosis and volume contraction. Spot urine chloride is 7 mmol/L. Which mechanism is most likely?

A. An obligatory diagnosis of diuretic-induced alkalosis B. Ongoing mineralocorticoid excess with high urine chloride C. Normal-anion-gap metabolic acidosis D. Respiratory acidosis due to hypoventilation E. Chloride depletion with saline-responsive metabolic alkalosis

View detailed teaching explanation and answered viva

Correct answer: E. Chloride depletion with saline-responsive metabolic alkalosis.

Why correct: Low urine chloride is consistent with appropriate renal chloride conservation due to extrarenal chloride loss such as vomiting. Correcting volume/chloride depletion typically helps renal bicarbonate excretion.

Why the other options are incorrect:

A. An obligatory diagnosis of diuretic-induced alkalosis: Active diuretics often elevate urine chloride and can make interpretation time-dependent.

B. Ongoing mineralocorticoid excess with high urine chloride: Mineralocorticoid-driven alkalosis generally has more persistent urinary chloride despite volume status.

C. Normal-anion-gap metabolic acidosis: The history and alkalosis are inconsistent with intestinal bicarbonate loss.

D. Respiratory acidosis due to hypoventilation: Urine chloride does not establish a respiratory disorder.

Exam pearl: Urine Cl under about 10–20 mmol/L favours chloride-responsive alkalosis in context.

Examiner trap: Urine chloride can be misleading with recent diuretic use or fluctuating intake.

Viva extension — Why is chloride important to bicarbonate handling?
Answer: Chloride availability and volume restoration facilitate renal bicarbonate excretion.

Question 33 of 40 • Lactic acidosis

Q33. A patient with massive haemorrhage develops pH 7.18, HCO3 10 mmol/L and a high anion gap. What mechanism most plausibly contributes?

A. Accumulation of lactate associated with tissue hypoperfusion B. Loss of stomach acid from vomiting C. Isolated chloride-rich fluid infusion D. Isolated primary respiratory alkalosis E. Selective renal HCO3 secretion without any other process

View detailed teaching explanation and answered viva

Correct answer: A. Accumulation of lactate associated with tissue hypoperfusion.

Why correct: Severe tissue hypoperfusion is associated with increased lactate production and impaired clearance, generating unmeasured anions and a high-anion-gap metabolic acidosis.

Why the other options are incorrect:

B. Loss of stomach acid from vomiting: Vomiting more often produces metabolic alkalosis.

C. Isolated chloride-rich fluid infusion: This more often causes normal-anion-gap hyperchloraemic acidosis.

D. Isolated primary respiratory alkalosis: It would have high pH and low PaCO2 rather than this metabolic pattern.

E. Selective renal HCO3 secretion without any other process: That does not account for the shock-associated high anion gap.

Exam pearl: In shock, assess perfusion and trends; lactate is a marker, not a complete diagnosis.

Examiner trap: Do not assume every high-gap acidosis is due to lactate; check ketones and other causes.

Viva extension — Why can lactate remain elevated after circulation improves?
Answer: Clearance may lag, with ongoing microcirculatory dysfunction, adrenergic effects or hepatic impairment.

Question 34 of 40 • Chronic respiratory acidosis compensation

Q34. A patient with longstanding hypoventilation has PaCO2 60 mmHg (baseline 40). Approximately how much can bicarbonate rise with chronic compensation?

A. About 2 mmol/L total B. About 15–20 mmol/L C. No change in bicarbonate D. About 7–8 mmol/L above usual bicarbonate E. About 1 mmol/L total

View detailed teaching explanation and answered viva

Correct answer: D. About 7–8 mmol/L above usual bicarbonate.

Why correct: With chronic respiratory acidosis, renal adaptation increases bicarbonate by approximately 3.5–4 mmol/L for every 10 mmHg PaCO2 rise. A 20-mmHg elevation predicts about 7–8 mmol/L increase.

Why the other options are incorrect:

A. About 2 mmol/L total: A 2-mmol rise approximates acute compensation for a 20-mmHg rise.

B. About 15–20 mmol/L: This greatly exceeds usual compensatory change.

C. No change in bicarbonate: The kidneys adapt over days to sustained hypercapnia.

E. About 1 mmol/L total: This substantially underestimates chronic renal adaptation.

Exam pearl: Acute respiratory acidosis ~+1 HCO3 per 10 CO2; chronic ~+4 per 10.

Examiner trap: Timing (hours vs days) changes expected bicarbonate.

Viva extension — Why does renal compensation take time?
Answer: Changes in bicarbonate reclamation and net acid excretion require renal cellular and transport adaptation.

Question 35 of 40 • Proximal RTA

Q35. A child with Fanconi syndrome loses bicarbonate in urine and has normal-anion-gap metabolic acidosis. What is the primary tubular defect?

A. Blockade of collecting-duct AQP2 B. Complete failure of urea filtration C. Reduced proximal bicarbonate reabsorption D. Failure of distal H+ secretion as sole defect E. Isolated aldosterone deficiency with hyperkalaemia

View detailed teaching explanation and answered viva

Correct answer: C. Reduced proximal bicarbonate reabsorption.

Why correct: Type 2 RTA results from impaired proximal reclamation of filtered bicarbonate. In Fanconi syndrome other proximal solute losses can coexist. Urine may become acidic once filtered bicarbonate load is reduced below the threshold.

Why the other options are incorrect:

A. Blockade of collecting-duct AQP2: This produces water concentrating defects, not primarily bicarbonate wasting.

B. Complete failure of urea filtration: This does not define RTA.

D. Failure of distal H+ secretion as sole defect: This is the main feature of type 1 distal RTA.

E. Isolated aldosterone deficiency with hyperkalaemia: This is more typical of type 4 RTA.

Exam pearl: Type 2 RTA: proximal bicarbonate reclamation defect, often part of Fanconi syndrome.

Examiner trap: Urine pH need not remain high after serum bicarbonate falls.

Viva extension — Name other Fanconi-type solute losses.
Answer: Phosphate, glucose (with normal blood glucose), uric acid and amino acids can be lost.

Question 36 of 40 • Distal RTA

Q36. An adult with recurrent calcium-phosphate stones and hypokalaemic normal-gap metabolic acidosis has urine pH 6.3. What is the likely mechanism?

A. Thick ascending limb water reabsorption B. Proximal loss of all filtered glucose only C. Impaired distal secretion of hydrogen ions D. High aldosterone with alkalosis E. SIADH

View detailed teaching explanation and answered viva

Correct answer: C. Impaired distal secretion of hydrogen ions.

Why correct: Type 1 distal RTA cannot appropriately acidify urine despite systemic acidosis, favouring alkaline urine, hypocitraturia, nephrocalcinosis or stones and often low potassium.

Why the other options are incorrect:

A. Thick ascending limb water reabsorption: The ascending limb is relatively impermeable to water.

B. Proximal loss of all filtered glucose only: Isolated glucosuria does not explain distal urinary acidification failure.

D. High aldosterone with alkalosis: Mineralocorticoid excess generally gives alkalosis, not distal RTA.

E. SIADH: Water retention causes hyponatraemia and is not the defining acid-base disorder.

Exam pearl: Distal RTA = alkaline urine in systemic acidosis plus stones/hypokalaemia.

Examiner trap: Check renal function and exclude urinary infection with urease organisms when interpreting alkaline urine.

Viva extension — What tends to promote stones in type 1 RTA?
Answer: Alkaline urine, low citrate and calcium-phosphate supersaturation.

Question 37 of 40 • Type 4 RTA

Q37. A diabetic patient taking an ACE inhibitor has K 5.8 mmol/L and mild normal-anion-gap metabolic acidosis. What mechanism is most consistent?

A. Reduced aldosterone action in the distal nephron B. Excess aldosterone secretion C. Isolated proximal glucose cotransport D. Pure vomiting-related chloride loss E. Classical type 1 distal RTA with hypokalaemia

View detailed teaching explanation and answered viva

Correct answer: A. Reduced aldosterone action in the distal nephron.

Why correct: Type 4 RTA commonly reflects deficient aldosterone or distal resistance to it. Impaired distal potassium and acid excretion yields hyperkalaemia with mild normal-gap metabolic acidosis.

Why the other options are incorrect:

B. Excess aldosterone secretion: It typically increases potassium loss and causes alkalosis.

C. Isolated proximal glucose cotransport: This would lead to glucosuria, not the characteristic hyperkalaemic acidosis.

D. Pure vomiting-related chloride loss: That tends to produce alkalosis and lower potassium.

E. Classical type 1 distal RTA with hypokalaemia: Type 1 RTA often features hypokalaemia rather than the hyperkalaemic type 4 phenotype.

Exam pearl: Type 4 RTA is the hyperkalaemic renal tubular acidosis.

Examiner trap: Consider medication, diabetic kidney disease and genuine AKI as competing causes.

Viva extension — How does hyperkalaemia worsen acid excretion?
Answer: It impairs renal ammonium production, contributing to decreased net acid excretion.

Question 38 of 40 • AKI recognition

Q38. After surgery a 70-kg patient produces 180 mL urine over 8 hours. Which statement best describes the urine-output criterion for AKI?

A. Only serum creatinine can define AKI B. Oliguria requires at least 24 hours in all adults C. A blocked urinary catheter can be excluded from the numbers alone D. It meets the oliguria criterion of less than 0.5 mL/kg/hour for more than 6 hours E. The rate is above 1 mL/kg/hour

View detailed teaching explanation and answered viva

Correct answer: D. It meets the oliguria criterion of less than 0.5 mL/kg/hour for more than 6 hours.

Why correct: Urine output is 180/(70 × 8) = 0.32 mL/kg/hour for 8 hours. This crosses the usual adult urine-output threshold for AKI identification and warrants prompt assessment.

Why the other options are incorrect:

A. Only serum creatinine can define AKI: Accepted AKI criteria also include sustained oliguria.

B. Oliguria requires at least 24 hours in all adults: The usual threshold is more than 6 hours, not necessarily a full day.

C. A blocked urinary catheter can be excluded from the numbers alone: Outflow obstruction or measurement error requires bedside assessment.

E. The rate is above 1 mL/kg/hour: The correct calculation gives 0.32 mL/kg/hour.

Exam pearl: Postoperative oliguria needs a time-adjusted, weight-adjusted calculation and clinical review.

Examiner trap: Low urine output is a warning sign, not proof of irreversible tubular necrosis.

Viva extension — What investigations should be considered initially?
Answer: Vitals, fluid balance, urine dipstick, U&E/creatinine trend, drug review, catheter assessment and imaging for obstruction when indicated.

Question 39 of 40 • Pre-renal physiology

Q39. An older surgical patient becomes hypotensive after haemorrhage, with concentrated urine and low urinary sodium before diuretics. Which renal response best explains this pattern?

A. Absent ADH with free-water diuresis B. Compensatory sodium and water conservation in reduced renal perfusion C. Osmotic diuresis as the only possible cause D. Complete absence of aldosterone E. Massive renal sodium wasting as the normal response

View detailed teaching explanation and answered viva

Correct answer: B. Compensatory sodium and water conservation in reduced renal perfusion.

Why correct: Lower effective circulating volume activates sympathetic tone, RAAS and ADH, enhancing tubular sodium and water reabsorption. These findings support a perfusion-related response but are not uniquely diagnostic.

Why the other options are incorrect:

A. Absent ADH with free-water diuresis: That would produce dilute high-volume urine.

C. Osmotic diuresis as the only possible cause: Osmotic diuresis tends to increase flow and is not required here.

D. Complete absence of aldosterone: That would impair sodium retention and often increase potassium.

E. Massive renal sodium wasting as the normal response: In uncomplicated hypoperfusion the kidneys conserve sodium.

Exam pearl: Prerenal urine indices are clues; diuretics and sepsis modify them.

Examiner trap: Volume-responsive physiology and intrinsic injury may coexist.

Viva extension — Why does urea sometimes rise disproportionately to creatinine with volume depletion?
Answer: Increased proximal urea reabsorption can accompany sodium and water conservation.

Question 40 of 40 • RRT indication

Q40. A patient with AKI has refractory K 7.0 mmol/L despite appropriate temporising measures, plus progressive pulmonary oedema. What is the most appropriate next principle?

A. Ignore pulmonary oedema if urine output persists B. Wait for a predetermined creatinine level C. Give potassium-rich maintenance fluid D. Use insulin indefinitely as sole definitive treatment E. Urgent specialist assessment for renal replacement therapy

View detailed teaching explanation and answered viva

Correct answer: E. Urgent specialist assessment for renal replacement therapy.

Why correct: Refractory life-threatening hyperkalaemia and uncontrolled volume overload are classic indications for urgent renal support, independent of a single creatinine threshold. Continue resuscitative care and appropriate temporising treatments while arranging definitive care.

Why the other options are incorrect:

A. Ignore pulmonary oedema if urine output persists: Persistent urine output does not protect against severe fluid overload or hyperkalaemia.

B. Wait for a predetermined creatinine level: RRT need is determined by clinical complications rather than one creatinine figure.

C. Give potassium-rich maintenance fluid: This can aggravate hyperkalaemia.

D. Use insulin indefinitely as sole definitive treatment: Insulin only shifts potassium; rebound is possible without removal.

Exam pearl: AEIOU is a memory aid; clinical refractoriness and urgency determine escalation.

Examiner trap: Dialysis need not wait for severe urea elevation if immediate complications exist.

Viva extension — Name other urgent complications suggesting RRT.
Answer: Severe refractory metabolic acidosis and certain uraemic manifestations, in addition to electrolyte and fluid overload.

Frequently asked questions

Do MRCS Part A questions use four or five options?

MRCS Part A uses five-option, single-best-answer questions. These 40 practice items follow that format and are original education questions, not official exam items.

Should I memorise every renal clearance formula?

Understand clearance, filtration fraction, calculated osmolality, anion gap and Winter's formula. Practise with units and clinical context rather than isolated arithmetic.

Does normal ECG exclude serious hyperkalaemia?

No. ECG findings are inconsistent and can be absent even with severe hyperkalaemia. Use urgent clinical and biochemical assessment and an appropriate emergency pathway.

Does low urine output always mean the patient requires more IV fluids?

No. Assess circulation, losses, medications, renal injury and obstruction. Some patients are overloaded or need urgent treatment of sepsis or other causes rather than blind fluid administration.

Can I copy these questions into another public recall bank?

These are original learning questions designed to practise published syllabus concepts. They should not be presented as recovered confidential examination questions or as an official college question paper.

Will my quiz progress sync between my phone and laptop?

No. The code saves only to this browser's localStorage. Use the same device and browser; switching devices or clearing website data will not sync results.

References and continue your MRCS preparation

These educational sources inform the clinical explanations. Uploaded MRCS recall collections helped identify subject priorities; any recall source-marked answer was not treated as automatically verified.

1. Intercollegiate MRCS — candidate guidance and syllabus — Official syllabus and exam preparation framework.
2. Physiology, GFR — NCBI Bookshelf — GFR physiology, clearance and renal function.
3. Renal blood flow and filtration — NCBI Bookshelf — PAH, filtration fraction and afferent/efferent responses.
4. NICE NG148 — Acute kidney injury — AKI detection and urgent clinical assessment.
5. NICE CG174 — Intravenous fluid therapy in adults — Fluid needs and maintenance principles in hospital.
6. UK Kidney Association — Acute hyperkalaemia — Current guideline hub and updated recommendations.
7. European clinical practice guideline — Hyponatraemia — Hyponatraemia assessment and safe correction principles.
8. Hyponatraemia treatment standard 2024 — NDT — Symptomatic hyponatraemia and avoiding rapid correction.

Previous lessons and useful articles

Article 01
Start MRCS Here Article 02
Complete Syllabus Article 03
Recommended Books Article 04
12-Week Plan Article 05
SBA Strategy Article 06
Common Mistakes Article 07
Anatomy Overview Article 10
Head & Neck Anatomy Article 11
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Editorial review: October 2026. Designed for examination learning; never a substitute for direct clinical assessment or institutional treatment protocols.