How to Solve MRCS Part A Single Best Answer Questions

FreeMedSite MRCS Academy · Article 05 · Updated October 2026

How to Solve MRCS Part A Single Best Answer Questions: Strategies, Examiner Traps & Practice MCQs

Many candidates know the topic but still choose the wrong option. This beginner-friendly guide shows how to read a clinical stem, identify the actual task, compare close distractors, manage time, and learn from mistakes. Then apply the method to 20 original five-option SBAs with detailed teaching explanations.

Paper 1 + Paper 220 explained clinical SBAsSaved practice progressMobile-first layout

What is an MRCS Part A SBA?

A single best answer (SBA) question gives a stem and five options (A–E). Select the best answer to the exact question asked—even when another option would sometimes be reasonable in a different clinical context.

180Paper 1: Applied Basic Sciences
3 hours · anatomy, physiology, pathology and more

120Paper 2: Principles of Surgery
2 hours · clinical care, trauma, perioperative topics and more

The two papers are on the same day, making 300 questions in five examination hours—an average of approximately 60 seconds per SBA. A pass requires meeting standards for both individual papers and the combined score. Official MRCS guidance indicates that wrong answers do not attract negative marks, so do not knowingly leave answers blank. Confirm operational details in the current exam instructions before your sitting.

Study principle: Recognising facts is not enough. You must decide which fact or clinical action answers this precise stem.

On this page

7-step SBA methodDecoding question keywordsCommon examiner trapsTiming and exam-day planError log and spaced revision20-question practice quizFrequently asked questionsSources and further reading

The 7-step method for solving MRCS SBAs

Use this method on learning-mode questions first; with repetition it becomes an efficient mental routine.

1

Read the final line first

Find whether the question asks for the most likely diagnosis, anatomical structure, investigation, immediate intervention, definitive treatment or an exception. That changes the answer.

2

Label the patient’s stability

Look for haemodynamic instability, respiratory failure, peritonitis, altered mental status, severe neurological compromise or threatened limb. An unstable patient usually needs action before elaborate investigations.

3

Extract three decisive clues

Age and timing matter, but prioritise discriminating clues: e.g., lateral versus medial to epigastric vessels; free fluid with shock; pain on passive stretch; pH and bicarbonate together.

4

Predict an answer before looking

Form a working diagnosis or management action. This reduces attraction to familiar-sounding distractors.

5

Eliminate options using one decisive reason

Cross out answers inconsistent with anatomy, physiology, timeline, patient stability or the question’s command. Avoid inventing data not in the stem.

6

Choose the best fit, not merely a true statement

Several options can be factually true. The correct choice is the one answering the precise task for this patient at this moment.

7

Commit, move, and review marked items

Do not spend several minutes on one SBA. Flag uncertain items, protect time for the remaining paper and return later if time permits.

Worked example: “A hypotensive trauma patient has ongoing bleeding and a positive FAST; what is the next definitive step?” The operative word is definitive. A CT scan may be useful in a stable patient, but ongoing instability shifts priority to haemorrhage control. This is not simply a question about what FAST detects.

Translate examiner keywords into decisions

Keyword in stem

What you should identify

Most likely

The diagnosis or mechanism that best explains all the supplied clues—not the rarest possibility.

Initial / immediate

First safe action, prioritising ABCDE, resuscitation and source control as appropriate.

Definitive

The intervention that treats the cause, not merely a temporising measure.

Best investigation

Choose a test appropriate to pretest probability, stability, and what result will change management.

Except / least likely

Find the one statement that does not fit. Mark the negative wording before reading options.

Most important risk

Distinguish the strongest or most decisive factor from merely associated features.

When clinical guidance is evolving, favour the latest relevant UK/Ireland recommendations and recognise that the appropriate action can differ between stable and unstable patients.

Ten common MRCS examiner traps

1. Initial versus definitive

Removing a tight dressing helps in compartment syndrome, but a clear established case still needs urgent fasciotomy.

2. Stable versus unstable

CT frequently guides stable trauma management; ongoing shock can demand urgent haemorrhage control.

3. Clinical signs versus investigations

A clinically suspected tension pneumothorax in an unstable patient should not wait for imaging.

4. Structures with similar names

Common fibular, deep fibular and superficial fibular nerve deficits are different.

5. Position words

Below the inguinal ligament differs from medial to the inferior epigastric vessels.

6. Base rate and diagnostic metrics

Sensitivity, specificity and positive predictive value have different denominators.

7. Primary versus compensation

Low PaCO₂ in metabolic acidosis may be appropriate respiratory compensation.

8. A correct fact in the wrong context

LMWH is used for prophylaxis—but not without considering active bleeding.

9. Early versus late signs

Palpable pulses do not exclude early acute compartment syndrome.

10. Absolute language

“Always”, “never” and “only” should trigger scrutiny; do not reject them automatically.

Watch for outdated exam-day advice: The 2021 official syllabus PDF includes older paper-answer-sheet instructions. MRCS Part A has been delivered through Pearson computer-based test centres since May 2022, so use the current candidate guidance for logistics.

Timing strategy: about one minute per question

The official durations allow roughly 60 seconds per question in both papers. That is an average, not a rigid stop-watch limit. Some simple recall questions may take 20–30 seconds, leaving more time for difficult clinical scenarios or calculations.

Practical first pass

• Answer clear questions immediately.
• For a hard SBA, remove implausible options and make a best choice.
• Use the exam software’s available review/flag feature as instructed.
• Do not allow one question to consume time intended for several others.

Second pass

• Return to flagged questions if time remains.
• Change an answer when a definite clue or corrected reasoning justifies it.
• Ensure every question has one answer recorded; the official content guide states there is no negative marking.
• Follow on-screen exam procedures rather than old pencil-answer-sheet advice.

Milestone

Suggested time checkpoint

Paper 1 · Q60

~60 minutes elapsed; ~120 minutes remain.

Paper 1 · Q120

~120 minutes elapsed; ~60 minutes remain.

Paper 2 · Q40

~40 minutes elapsed; ~80 minutes remain.

Paper 2 · Q80

~80 minutes elapsed; ~40 minutes remain.

Checkpoints are personal pacing suggestions, not official exam instructions. Reserve part of the remaining time to revisit uncertain questions when possible.

Turn wrong answers into an error log

After an SBA block, classify why you missed each question. Reading the correct letter alone does not build reliable recall.

Error type

How to correct it

Knowledge gap

Read the core concept, draw the anatomy or explain the mechanism in your own words.

Stem misread

Underline “initial”, “definitive”, “except”, time course or instability.

Two close options

Write one decisive difference between them and revisit a contrasting example.

Guideline mismatch

Check an up-to-date official guideline rather than relying on an old recall.

Time pressure

Use short mixed timed blocks and practise committing to the best answer.

Suggested spaced review

Revisit mistakes after 1 day, 3 days, 7 days and 21 days. These are practical revision intervals, not a claim about an official MRCS learning schedule. Retest the underlying principle from a different clinical angle.

Practice 20 original MRCS-style SBAs

Questions span anatomy, physiology, pathology, pharmacology, trauma, perioperative care, ethics, diagnostics and surgery. They are independently written educational questions, not disclosed exam items. Each has five options and a teaching explanation of every option, an examiner trap and an answered viva extension.

How it works: Select A–E, then press Check answer. Read the explanation and use Next question. Your selection, marked result and current question are saved automatically in this browser. The browser does not send this quiz data to FreeMedSite or synchronise it across devices.

0 / 20Checked

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—Accuracy

Questions remain available even if browser storage is blocked.

Question 1 of 20

Applied anatomy

Q1. A 66-year-old woman presents with a tender groin swelling located inferior to the inguinal ligament and medial to the femoral vein. Which anatomical diagnosis best fits these findings?

A. Direct inguinal hernia B. Indirect inguinal hernia C. Saphena varix D. Femoral hernia E. Enlarged inguinal lymph node

Answer, full explanation and viva extension

Correct answer: D. Femoral hernia

Why D is correct: The femoral canal lies medial to the femoral vein and below the inguinal ligament; this is the classic site of a femoral hernia. It has a relatively narrow neck and a higher risk of strangulation.

Why the other options are incorrect

A. Direct inguinal hernia: A direct inguinal hernia emerges through Hesselbach’s triangle above the inguinal ligament.

B. Indirect inguinal hernia: An indirect inguinal hernia passes through the deep inguinal ring, also above the inguinal ligament.

C. Saphena varix: A saphena varix may mimic a groin hernia but arises from the saphenofemoral junction and the described femoral canal relationship is more specific.

E. Enlarged inguinal lymph node: Inguinal lymph nodes can produce swellings but are not defined by the femoral canal landmark.

Exam pearl: Femoral hernia: below the inguinal ligament, medial to the femoral vein.

Examiner trap: “Medial to the inferior epigastric vessels” distinguishes direct from indirect inguinal hernias; it does not define a femoral hernia.

Viva extension (answered): What forms the medial boundary of the femoral ring? The lacunar ligament.

Q2. A 28-year-old patient develops foot drop after a fracture around the fibular neck. Dorsiflexion and eversion are weak, but plantarflexion is preserved. Which nerve has most likely been injured?

A. Tibial nerve B. Common fibular (peroneal) nerve C. Femoral nerve D. Obturator nerve E. Saphenous nerve

Answer, full explanation and viva extension

Correct answer: B. Common fibular (peroneal) nerve

Why B is correct: The common fibular nerve winds around the fibular neck; injury affects the deep fibular branch (dorsiflexion) and superficial fibular branch (eversion), producing foot drop.

Why the other options are incorrect

A. Tibial nerve: The tibial nerve primarily supplies the posterior leg compartment, responsible for plantarflexion.

C. Femoral nerve: Femoral nerve injury impairs knee extension and can reduce patellar reflex.

D. Obturator nerve: Obturator nerve injury impairs thigh adduction.

E. Saphenous nerve: The saphenous nerve is sensory and would not explain foot drop.

Exam pearl: Fibular neck trauma plus weak dorsiflexion and eversion = common fibular nerve.

Examiner trap: The deep fibular nerve alone causes dorsiflexion weakness but usually spares eversion.

Viva extension (answered): Which branch innervates tibialis anterior? The deep fibular nerve.

Q3. Immediately after acute haemorrhage, before major fluid replacement, which response is most likely to help preserve arterial pressure?

A. Decreased sympathetic outflow B. Increased vagal stimulation of the sinoatrial node C. Widespread arteriolar vasodilatation D. Reduction in heart rate E. Increased baroreceptor-mediated sympathetic activity

Answer, full explanation and viva extension

Correct answer: E. Increased baroreceptor-mediated sympathetic activity

Why E is correct: Reduced baroreceptor firing increases sympathetic tone, producing tachycardia, increased contractility and peripheral vasoconstriction.

Why the other options are incorrect

A. Decreased sympathetic outflow: Sympathetic outflow increases, not decreases, when arterial pressure falls.

B. Increased vagal stimulation of the sinoatrial node: Vagal stimulation slows the heart and does not provide the principal compensatory response.

C. Widespread arteriolar vasodilatation: Widespread vasodilatation would reduce systemic vascular resistance and worsen hypotension.

D. Reduction in heart rate: Tachycardia is more typical than bradycardia in uncomplicated early haemorrhagic shock.

Exam pearl: Acute blood loss → ↓ baroreceptor firing → ↑ sympathetic tone → tachycardia and vasoconstriction.

Examiner trap: A normal blood pressure early in haemorrhage does not exclude significant blood loss.

Viva extension (answered): What happens to pulse pressure in compensated blood loss? It can narrow as stroke volume falls.

Q4. An unwell surgical patient has arterial blood gas values: pH 7.25, PaCO₂ 25 mmHg and bicarbonate 11 mmol/L. What is the primary acid–base disturbance?

A. Metabolic acidosis with respiratory compensation B. Primary respiratory acidosis C. Metabolic alkalosis D. Primary respiratory alkalosis E. Mixed metabolic and respiratory alkalosis

Answer, full explanation and viva extension

Correct answer: A. Metabolic acidosis with respiratory compensation

Why A is correct: The low pH indicates acidaemia and the markedly low bicarbonate identifies primary metabolic acidosis. The low PaCO₂ is compensatory hyperventilation; Winter’s formula predicts approximately 24.5 ± 2 mmHg.

Why the other options are incorrect

B. Primary respiratory acidosis: Primary respiratory acidosis has elevated PaCO₂, not 25 mmHg.

C. Metabolic alkalosis: Metabolic alkalosis typically raises bicarbonate and pH.

D. Primary respiratory alkalosis: A primary respiratory alkalosis causes an elevated pH; the pH here is low.

E. Mixed metabolic and respiratory alkalosis: Both alkalosis processes would raise pH, contrary to the measured acidaemia.

Exam pearl: Low pH + low HCO₃⁻ = metabolic acidosis; assess compensation rather than assuming a second primary disorder.

Examiner trap: Do not label a low PaCO₂ as primary respiratory alkalosis without checking pH and bicarbonate.

Viva extension (answered): What is Winter’s formula? Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2 mmHg.

Q5. A 72-year-old woman is admitted for abdominal surgery. She has limited mobility and a history of venous thromboembolism, but also a recent gastrointestinal bleed. What is the most appropriate principle when planning thromboprophylaxis?

A. Give full-dose anticoagulation to every patient B. Avoid any risk assessment because surgery itself is sufficient C. Assess both thrombosis and bleeding risk and individualise mechanical and pharmacological prophylaxis D. Offer aspirin alone to every surgical patient E. Delay all measures until postoperative day seven

Answer, full explanation and viva extension

Correct answer: C. Assess both thrombosis and bleeding risk and individualise mechanical and pharmacological prophylaxis

Why C is correct: NICE NG89 recommends assessing VTE and bleeding risk in surgical patients and choosing measures according to that balance; a recent bleed can alter pharmacological choices.

Why the other options are incorrect

A. Give full-dose anticoagulation to every patient: Therapeutic anticoagulation is not the same as prophylaxis and carries bleeding risk.

B. Avoid any risk assessment because surgery itself is sufficient: Formal risk assessment is required because thrombotic and bleeding risks differ by patient and procedure.

D. Offer aspirin alone to every surgical patient: Aspirin is not universal first-line prophylaxis across all surgical procedures.

E. Delay all measures until postoperative day seven: Appropriate mechanical prophylaxis and mobilisation can begin early; seven-day blanket delays are unsafe.

Exam pearl: VTE prophylaxis = assess clotting risk AND bleeding risk, then choose and review the intervention.

Examiner trap: Prophylactic LMWH is not automatically appropriate in a patient with active or significant recent bleeding.

Viva extension (answered): When should risk be reassessed? At consultant review and whenever clinical condition changes.

Q6. A large contaminated traumatic skin wound is left open and gradually heals with substantial granulation tissue and wound contraction. This is best described as healing by which mechanism?

A. Primary intention B. Secondary intention C. Healing without inflammation D. Epithelial regeneration without granulation E. Immediate collagen maturation

Answer, full explanation and viva extension

Correct answer: B. Secondary intention

Why B is correct: Secondary intention is typical for open wounds with tissue loss; granulation tissue fills the defect and myofibroblasts contribute to contraction.

Why the other options are incorrect

A. Primary intention: Primary intention involves approximation of relatively clean wound edges, usually with limited tissue loss.

C. Healing without inflammation: The inflammatory phase is important for wound healing and cannot be excluded.

D. Epithelial regeneration without granulation: Granulation tissue is prominent when the defect is large and open.

E. Immediate collagen maturation: Collagen remodelling occurs over weeks to months, not immediately.

Exam pearl: Secondary intention = open defect + granulation tissue + contraction.

Examiner trap: Delayed primary closure (tertiary intention) is not the same as leaving a wound to heal entirely by secondary intention.

Viva extension (answered): Which cells mediate wound contraction? Myofibroblasts.

Q7. A patient with a high-output ileostomy develops dehydration and a normal-anion-gap metabolic acidosis. Which process most directly explains the acid–base disturbance?

A. Renal retention of bicarbonate B. Vomiting-related loss of gastric acid C. Carbon dioxide retention D. Excess aldosterone causing hydrogen loss E. Gastrointestinal loss of bicarbonate-rich fluid

Answer, full explanation and viva extension

Correct answer: E. Gastrointestinal loss of bicarbonate-rich fluid

Why E is correct: Intestinal fluid losses can deplete bicarbonate and lead to a hyperchloraemic normal-anion-gap metabolic acidosis.

Why the other options are incorrect

A. Renal retention of bicarbonate: Bicarbonate retention would favour metabolic alkalosis.

B. Vomiting-related loss of gastric acid: Loss of gastric hydrochloric acid from vomiting classically causes metabolic alkalosis.

C. Carbon dioxide retention: Carbon dioxide retention produces primary respiratory acidosis.

D. Excess aldosterone causing hydrogen loss: Hydrogen-ion loss tends towards alkalosis rather than the described metabolic acidosis.

Exam pearl: Diarrhoea or intestinal bicarbonate loss → normal-anion-gap metabolic acidosis; vomiting often → metabolic alkalosis.

Examiner trap: Always identify which GI fluid is being lost rather than assuming all GI losses produce alkalosis.

Viva extension (answered): What other common metabolic issue may accompany high-output stoma? Sodium and water depletion, often with hypomagnesaemia.

Q8. A 40-year-old man is brought in after a road-traffic collision. Despite blood-product resuscitation, he remains profoundly hypotensive. A focused ultrasound examination shows free intraperitoneal fluid. What is the most appropriate next definitive step?

A. Send him for a prolonged MRI examination B. Observe for six hours and repeat the scan C. Discharge after an apparently normal pelvic radiograph D. Urgent operative haemorrhage control with continued resuscitation E. Delay intervention until a routine CT report is available

Answer, full explanation and viva extension

Correct answer: D. Urgent operative haemorrhage control with continued resuscitation

Why D is correct: Persistent instability with suspected intra-abdominal haemorrhage requires immediate definitive haemorrhage control (usually emergency laparotomy) while damage-control resuscitation continues.

Why the other options are incorrect

A. Send him for a prolonged MRI examination: MRI is not appropriate for an unstable haemorrhaging trauma patient.

B. Observe for six hours and repeat the scan: Observation risks ongoing exsanguination in an unstable patient.

C. Discharge after an apparently normal pelvic radiograph: A pelvic radiograph cannot negate positive evidence of intra-abdominal bleeding.

E. Delay intervention until a routine CT report is available: CT is valuable when the patient is stable or responding sufficiently to resuscitation, but must not delay life-saving haemorrhage control in the non-responder.

Exam pearl: Unstable + positive intraperitoneal FAST + ongoing haemorrhage → act for haemorrhage control, not a routine CT delay.

Examiner trap: FAST detects fluid; a negative FAST does NOT reliably exclude retroperitoneal or intra-abdominal bleeding.

Viva extension (answered): What does eFAST additionally assess? Pleural fluid and pneumothorax, alongside abdominal/pericardial fluid.

Q9. Following an enclosed-space fire, a patient has progressive hoarseness, audible stridor and increasing work of breathing. What should the initial management prioritise?

A. Urgent assessment and early definitive airway protection B. Wait for chest X-ray changes before intervening C. Treat with oral antihistamines and discharge D. Obtain a routine spirometry test E. Give only a topical dressing to the facial burn

Answer, full explanation and viva extension

Correct answer: A. Urgent assessment and early definitive airway protection

Why A is correct: Progressive stridor and voice change suggest evolving upper-airway oedema; an experienced airway team should secure the airway early before access becomes difficult.

Why the other options are incorrect

B. Wait for chest X-ray changes before intervening: An initially normal chest X-ray does not exclude inhalational injury or upper-airway obstruction.

C. Treat with oral antihistamines and discharge: These signs are not typical of an uncomplicated allergic reaction and discharge is unsafe.

D. Obtain a routine spirometry test: Spirometry is not an emergency investigation for impending airway compromise.

E. Give only a topical dressing to the facial burn: Burn dressings do not treat threatened airway obstruction.

Exam pearl: Progressive hoarseness/stridor after smoke exposure = anticipate impending airway obstruction.

Examiner trap: Singed nasal hairs alone are not a sufficient indication for intubation; progressive airway compromise is the decisive finding.

Viva extension (answered): Why can airway intubation become harder over time? Thermal and inflammatory oedema progresses.

Q10. A patient receives unfractionated heparin for treatment of a venous thromboembolism. What is its principal mechanism of anticoagulant action?

A. Direct irreversible inhibition of platelet cyclooxygenase B. Inhibition of vitamin K epoxide reductase C. Potentiation of antithrombin-mediated inhibition of thrombin and factor Xa D. Direct oral inhibition of factor Xa E. Suppression of hepatic fibrinogen synthesis

Answer, full explanation and viva extension

Correct answer: C. Potentiation of antithrombin-mediated inhibition of thrombin and factor Xa

Why C is correct: Heparin binds antithrombin, greatly increasing its ability to inhibit thrombin (factor IIa) and factor Xa.

Why the other options are incorrect

A. Direct irreversible inhibition of platelet cyclooxygenase: Irreversible platelet cyclooxygenase inhibition is the principal action of aspirin.

B. Inhibition of vitamin K epoxide reductase: Vitamin K epoxide reductase inhibition is the mechanism of warfarin.

D. Direct oral inhibition of factor Xa: Direct Xa inhibitors include apixaban and rivaroxaban, not unfractionated heparin.

E. Suppression of hepatic fibrinogen synthesis: Unfractionated heparin does not act chiefly by preventing hepatic fibrinogen synthesis.

Exam pearl: Unfractionated heparin acts through antithrombin; warfarin inhibits vitamin K recycling.

Examiner trap: LMWH has relatively greater anti-Xa than anti-IIa activity compared with unfractionated heparin.

Viva extension (answered): What is the standard reversal agent for unfractionated heparin? Protamine sulfate.

Q11. A patient with an intact level of consciousness refuses an elective operation after a clear explanation of its risks. She can understand, retain, use or weigh the relevant information, and communicate her decision. Which statement is most accurate?

A. Refusal proves she lacks capacity B. Her surgeon can override her choice if treatment is beneficial C. She has decision-making capacity and her informed refusal should normally be respected D. A relative automatically becomes the decision-maker E. Capacity can be assessed only by a psychiatrist

Answer, full explanation and viva extension

Correct answer: C. She has decision-making capacity and her informed refusal should normally be respected

Why C is correct: Capacity is decision-specific and time-specific; the ability to understand, retain, weigh and communicate satisfies the functional assessment in this scenario.

Why the other options are incorrect

A. Refusal proves she lacks capacity: Disagreeing with medical advice is not evidence of incapacity.

B. Her surgeon can override her choice if treatment is beneficial: A capacitous adult may decline treatment even when clinicians think it is beneficial.

D. A relative automatically becomes the decision-maker: Relatives do not automatically replace an adult who has capacity.

E. Capacity can be assessed only by a psychiatrist: Capacity can be assessed by appropriately trained clinicians; psychiatry input is not automatically required.

Exam pearl: An unwise decision alone does not mean lack of capacity.

Examiner trap: Capacity and consent are different: the patient may have capacity yet decline a proposed intervention.

Viva extension (answered): What four functional abilities are assessed? Understand, retain, use/weigh, communicate.

Q12. An invasive carcinoma is located in the upper outer quadrant of the breast. Which lymph-node basin is most commonly the first major regional drainage site?

A. Ipsilateral axillary lymph nodes B. Popliteal lymph nodes C. Deep cervical lymph nodes D. Para-aortic lymph nodes E. Inguinal lymph nodes

Answer, full explanation and viva extension

Correct answer: A. Ipsilateral axillary lymph nodes

Why A is correct: Most breast lymph drains to the ipsilateral axillary lymph nodes, particularly from the upper outer quadrant.

Why the other options are incorrect

B. Popliteal lymph nodes: Popliteal nodes drain parts of the leg and foot.

C. Deep cervical lymph nodes: Deep cervical nodes are primarily related to drainage of the head and neck.

D. Para-aortic lymph nodes: Para-aortic nodes are not the principal first regional basin for the upper outer breast.

E. Inguinal lymph nodes: Inguinal nodes drain the lower limb, external genitalia and parts of the lower abdominal wall.

Exam pearl: The axillary nodes receive most breast lymph drainage; internal mammary drainage is also clinically important.

Examiner trap: Lymphatic drainage is not identical for all breast regions; medial quadrants may drain toward parasternal nodes.

Viva extension (answered): What is a sentinel lymph node? The first lymph node(s) receiving drainage from a tumour-bearing area.

Q13. On postoperative day four after abdominal surgery, a haemodynamically stable patient develops unilateral calf swelling and tenderness. Deep vein thrombosis is suspected. Which imaging test is the standard first-line investigation when indicated by the diagnostic pathway?

A. Plain ankle radiograph B. Routine CT of the brain C. Abdominal ultrasound D. Compression ultrasound of the affected leg veins E. Diagnostic knee arthroscopy

Answer, full explanation and viva extension

Correct answer: D. Compression ultrasound of the affected leg veins

Why D is correct: Duplex/compression venous ultrasonography is the standard first-line imaging test for suspected lower-limb DVT, with choice and timing guided by pretest probability and local pathways.

Why the other options are incorrect

A. Plain ankle radiograph: A radiograph does not assess venous thrombosis.

B. Routine CT of the brain: Brain CT has no routine role in isolated suspected leg DVT.

C. Abdominal ultrasound: Abdominal ultrasound would not directly assess the symptomatic deep calf/proximal leg veins.

E. Diagnostic knee arthroscopy: Arthroscopy is invasive and unrelated to diagnosing DVT.

Exam pearl: Suspected lower-limb DVT → clinical probability assessment and compression ultrasonography when indicated.

Examiner trap: D-dimer is often elevated after surgery; it is not a definitive diagnostic test in this setting.

Viva extension (answered): What serious complication must be screened for? Pulmonary embolism: dyspnoea, pleuritic pain, tachycardia or hypoxaemia.

Q14. A patient has severe watery diarrhoea for four days and develops a metabolic acidosis with normal calculated anion gap. What is the most likely mechanism?

A. Retention of gastric hydrochloric acid B. Increased ketone production C. Accumulation of lactate from prolonged convulsions D. Ingestion of methanol E. Loss of bicarbonate from the gastrointestinal tract

Answer, full explanation and viva extension

Correct answer: E. Loss of bicarbonate from the gastrointestinal tract

Why E is correct: Loss of bicarbonate-rich intestinal fluid leads to hyperchloraemic, normal-anion-gap metabolic acidosis.

Why the other options are incorrect

A. Retention of gastric hydrochloric acid: Retaining gastric hydrochloric acid is not the characteristic mechanism of diarrhoeal acidosis.

B. Increased ketone production: Ketoacidosis typically raises the anion gap.

C. Accumulation of lactate from prolonged convulsions: Lactic acidosis typically raises the anion gap.

D. Ingestion of methanol: Methanol toxicity produces a high-anion-gap metabolic acidosis.

Exam pearl: High anion gap: add unmeasured acids. Normal anion gap: often bicarbonate loss or renal tubular acidosis.

Examiner trap: Diarrhoea causes metabolic acidosis; prolonged vomiting typically causes metabolic alkalosis.

Viva extension (answered): What is the usual formula for anion gap? Na⁺ − (Cl⁻ + HCO₃⁻), with reference ranges dependent on laboratory method and albumin.

Q15. In a study of a new test, 100 patients truly have a disease. The test is positive in 90 of these patients and negative in 10. What is the sensitivity of the test?

A. 10% B. 90% C. 50% D. 81% E. 100%

Answer, full explanation and viva extension

Correct answer: B. 90%

Why B is correct: Sensitivity = true positives ÷ (true positives + false negatives) = 90 ÷ (90 + 10) = 90%.

Why the other options are incorrect

A. 10%: 10% is the false-negative proportion among those with disease in this example.

C. 50%: 50% has no support from the provided figures.

D. 81%: 81% might come from inappropriate multiplication of 0.9 by itself; sensitivity is not squared.

E. 100%: 100% would mean all diseased patients tested positive, but 10 tested negative.

Exam pearl: Sensitivity answers: “Of patients who truly have the disease, how many test positive?”

Examiner trap: Sensitivity is not positive predictive value; PPV depends on true positives and false positives and is affected by prevalence.

Viva extension (answered): What is specificity? TN ÷ (TN + FP): probability of a negative test in those without disease.

Q16. A ventilated trauma patient develops sudden severe hypoxia, hypotension, absent left-sided breath sounds and increasing difficulty delivering positive-pressure ventilation. Tension pneumothorax is strongly suspected. What is the most appropriate immediate response?

A. Wait for a portable chest X-ray B. Arrange outpatient pulmonary function testing C. Increase ventilator pressures and observe D. Administer only intravenous diuretics E. Immediate chest decompression followed by definitive drainage

Answer, full explanation and viva extension

Correct answer: E. Immediate chest decompression followed by definitive drainage

Why E is correct: Prompt decompression is lifesaving; the approach depends on setting, training and protocol, followed by a chest drain when indicated.

Why the other options are incorrect

A. Wait for a portable chest X-ray: Imaging must not delay treatment when tension pneumothorax is clinically suspected in an unstable patient.

B. Arrange outpatient pulmonary function testing: Pulmonary function testing is irrelevant to an acute life-threatening emergency.

C. Increase ventilator pressures and observe: Higher positive-pressure ventilation can aggravate tension physiology.

D. Administer only intravenous diuretics: Diuretics do not release intrapleural pressure.

Exam pearl: Tension pneumothorax is a clinical emergency: decompress first in an unstable patient.

Examiner trap: Do not wait for tracheal deviation: it may be absent and is often late.

Viva extension (answered): Why does hypotension occur? Raised intrathoracic pressure reduces venous return and cardiac output.

Q17. A 78-year-old man with atrial fibrillation develops sudden severe abdominal pain. Examination shows a relatively soft abdomen despite marked pain, and lactate subsequently rises. Which diagnosis should be considered urgently?

A. Uncomplicated chronic constipation B. Acute cholecystitis C. Acute mesenteric ischaemia D. Irritable bowel syndrome E. Chronic pancreatitis

Answer, full explanation and viva extension

Correct answer: C. Acute mesenteric ischaemia

Why C is correct: Embolic occlusion of a mesenteric artery is associated with atrial fibrillation; early mesenteric ischaemia often presents with pain out of proportion to examination.

Why the other options are incorrect

A. Uncomplicated chronic constipation: Constipation does not usually produce abrupt severe pain with ischaemic physiology.

B. Acute cholecystitis: Cholecystitis generally causes localised right upper quadrant pain and tenderness.

D. Irritable bowel syndrome: IBS is a chronic functional condition and does not explain acute severe pain with physiological deterioration.

E. Chronic pancreatitis: Chronic pancreatitis usually has a recurrent or chronic pain pattern and characteristic pancreatic features.

Exam pearl: Sudden pain out of proportion to abdominal findings + AF = think acute mesenteric ischaemia.

Examiner trap: A normal early lactate cannot rule out mesenteric ischaemia; urgent evaluation is required.

Viva extension (answered): Which major vessel is commonly involved in embolic acute mesenteric ischaemia? The superior mesenteric artery.

Q18. During an appendicectomy, the surgeon identifies the mesoappendix. The appendicular artery most commonly arises from which arterial branch?

A. Right gastroepiploic artery B. Ileocolic artery C. Left colic artery D. Inferior rectal artery E. Splenic artery

Answer, full explanation and viva extension

Correct answer: B. Ileocolic artery

Why B is correct: The appendicular artery is usually a branch of the ileocolic artery (a branch of the superior mesenteric artery).

Why the other options are incorrect

A. Right gastroepiploic artery: The right gastroepiploic artery supplies the greater curvature of the stomach.

C. Left colic artery: The left colic artery arises from the inferior mesenteric system and supplies the descending colon.

D. Inferior rectal artery: The inferior rectal artery supplies the anal canal region.

E. Splenic artery: The splenic artery supplies the spleen and contributes to pancreatic and gastric vessels.

Exam pearl: Appendicular artery ← ileocolic artery ← superior mesenteric artery.

Examiner trap: The appendix has variable positional anatomy, but its arterial supply usually remains in the ileocolic territory.

Viva extension (answered): What is the anatomical landmark used to locate the appendix at the caecum? Convergence of the three taeniae coli.

Q19. Histology confirms a primary cutaneous melanoma. Which feature is the principal measurement used to define the thickness-based T category for the primary tumour?

A. Patient age B. Mitotic count alone C. Maximum horizontal tumour diameter D. Breslow tumour thickness E. Number of clinically palpable lymph nodes

Answer, full explanation and viva extension

Correct answer: D. Breslow tumour thickness

Why D is correct: Breslow thickness measures melanoma depth in millimetres and is central to primary tumour T categorisation, together with ulceration status.

Why the other options are incorrect

A. Patient age: Age affects overall prognosis but is not the primary thickness measurement of the tumour.

B. Mitotic count alone: Mitotic activity can be prognostically relevant but is not the principal measurement used for the T thickness category.

C. Maximum horizontal tumour diameter: Horizontal diameter is not the Breslow measurement.

E. Number of clinically palpable lymph nodes: Node involvement contributes to N stage rather than primary tumour thickness (T).

Exam pearl: Melanoma T staging relies on Breslow thickness and ulceration.

Examiner trap: Clark level describes anatomical levels of invasion and is not a substitute for Breslow thickness.

Viva extension (answered): What is Breslow thickness measured from? The top of the granular layer (or base of an ulcer) to the deepest malignant cell.

Q20. Six hours after a tibial fracture, an awake patient has escalating leg pain despite analgesia, extreme pain on passive toe stretch and a tense anterior compartment. Compartment syndrome is clinically evident. What is the definitive urgent treatment?

A. Emergency fasciotomy after immediately relieving external constriction and arranging surgical care B. Increase opioid dose and review tomorrow C. Elevate the limb well above the level of the heart D. Wait for MRI before intervention E. Apply a tighter circumferential cast

Answer, full explanation and viva extension

Correct answer: A. Emergency fasciotomy after immediately relieving external constriction and arranging surgical care

Why A is correct: A clear diagnosis of acute compartment syndrome requires urgent operative decompression. Immediately split/remove constrictive dressings or casts, keep the limb near heart level, support perfusion and proceed to fasciotomy without diagnostic delay.

Why the other options are incorrect

B. Increase opioid dose and review tomorrow: Analgesia alone does not reverse ischaemic compartment pressure.

C. Elevate the limb well above the level of the heart: Excessive elevation may reduce arterial perfusion to threatened tissues.

D. Wait for MRI before intervention: MRI is not required for a clinically clear limb-threatening emergency.

E. Apply a tighter circumferential cast: A tighter cast worsens pressure and perfusion compromise.

Exam pearl: Disproportionate pain and pain with passive stretch are early red flags; urgent fasciotomy is definitive when ACS is clear.

Examiner trap: Pulses may remain palpable; their presence does not exclude compartment syndrome.

Viva extension (answered): How many compartments of the leg must be released? Four: anterior, lateral, superficial posterior and deep posterior.

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Frequently asked questions

Does MRCS Part A have negative marking?

The official MRCS content guide states that wrong answers do not have marks deducted. Select an option for every item; check updated examination guidance before attending your sitting.

How long should I spend on an SBA?

The five-hour, 300-question format averages one minute per question. Some will be faster; others need extra time. Use timed practice to learn your personal balance.

Should I read the options before the clinical scenario?

Reading the last line first is often helpful because it tells you the task. Read the stem next and form your own provisional answer before examining all five options.

What should I do when two options both seem correct?

Check the exact task and discriminators: patient stability, timing, location, physiology and whether the question asks for an initial or definitive action. The most appropriate answer should fit the entire vignette.

Are these 20 questions actual past MRCS questions?

No. They are original teaching questions written around syllabus concepts. They are not exam recalls or official College questions.

Will my answers still be there next week?

Yes, normally, if you return to the same URL on the same browser and storage is enabled. Private browsing, clearing site data, browser restrictions, or another device can prevent restoration. The Reset button removes the stored quiz data.

Is this quiz enough to prepare for MRCS Part A?

No. It teaches the SBA method. Use it alongside systematic syllabus coverage, a quality question bank, source-based explanations and timed full-length mocks.

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Official MRCS candidate guidance

References and editorial note

1. Intercollegiate MRCS — Candidate Guidance. Current format and Pearson delivery updates.
2. Intercollegiate Committee for Basic Surgical Examinations — Guide to MRCS (August 2021). Five-option SBA format, syllabus, exam marking and worked sample questions. Paper-answer-sheet text in this older guide is obsolete for Pearson CBT logistics.
3. Royal College of Surgeons of Edinburgh — MRCS Part A Details. Paper structure and combined/individual standards.
4. NICE NG39 — Major trauma: assessment and initial management. Trauma haemorrhage and investigation priorities.
5. NICE NG89 — VTE risk assessment and prophylaxis. Surgical VTE and bleeding risk assessment.

Editorial note: This is an independent educational article, not an official Royal College course or guaranteed representation of question difficulty. Questions are original, authored to illustrate examinable principles. Guidelines change; always check current official recommendations and local clinical protocols. Reviewed 9 October 2026.

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