Daily Practice - Clinical Quiz #4

A 48-year-old woman presents to the emergency department with persistent vomiting for 24 hours after eating a hamburger. She reports multiple episodes of bilious emesis and decreased oral intake. Her medical history includes osteoarthritis and hypertension, and she is taking ibuprofen and lisinopril.

📋 Examination & Laboratory Findings

On examination: Dry mucous membranes, decreased skin turgor, tachycardia (110/min), and low blood pressure (101/62 mm Hg).

Na⁺: 136 mEq/L K⁺: 2.8 mEq/L Cl⁻: 85 mEq/L HCO₃⁻: 29 mEq/L BUN: 40 mg/dL Creatinine: 1.7 mg/dL FENa: < 1% Urine Osm: 650 mOsm/kg

👉 Question: Renal Pathophysiology

Which of the following best explains this patient’s renal findings?

A. Renal papillary necrosis

B. Hemolytic uremic syndrome

C. Hepatorenal syndrome

D. Acute interstitial nephritis

E. Prerenal azotemia

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✅ Correct Answer:

E. Prerenal azotemia.

📚 Expert Detailed Answer and Rationale:

This patient’s presentation—volume depletion from vomiting, tachycardia, hypotension, and dry mucous membranes, combined with a BUN:creatinine ratio > 20, FENa < 1%, and concentrated urine (osmolality > 500 mOsm/kg)—is characteristic of prerenal acute kidney injury (AKI), also known as prerenal azotemia.

E. ✅ Prerenal azotemia – In prerenal AKI, decreased renal perfusion (due to dehydration) leads to a decreased GFR. The kidneys compensate by increasing sodium and water reabsorption in an attempt to restore volume. This results in highly concentrated urine with very little sodium. The patient's use of ibuprofen (an NSAID, which constricts afferent arterioles) and lisinopril (an ACE inhibitor, which dilates efferent arterioles) further impairs the kidney's ability to maintain GFR in a low-volume state.

A, B, C, D. ❌ Other AKI types – These represent intrinsic renal or other systemic conditions. Acute interstitial nephritis is often drug-induced but typically presents with rash, fever, and eosinophils in the urine. Renal papillary necrosis is associated with NSAID use but often presents with hematuria and flank pain. HUS involves a triad of hemolytic anemia, thrombocytopenia, and AKI, usually after a diarrheal illness. Hepatorenal syndrome occurs in advanced liver disease. None of these fit the classic picture of volume depletion with appropriate renal compensation seen here.

🧠 High-Yield Points:

• 💡 Hallmark of Prerenal AKI: A physiological response to renal hypoperfusion.
• 💡 BUN/Cr Ratio: > 20:1, as BUN is reabsorbed with water while creatinine is not.
• 💡 Fractional Excretion of Sodium (FENa): < 1% because the tubules are avidly reabsorbing sodium.
• 💡 Urine Osmolality: > 500 mOsm/kg because ADH is high, leading to maximal water reabsorption.
• 💡 If left untreated, prerenal AKI can progress to ischemic acute tubular necrosis (ATN), which is an intrinsic renal failure.

📖 Read More on: Acute Kidney Injury

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💡 Clinical Challenge / Follow-Up:

Imagine the same patient does not receive timely fluid resuscitation. 48 hours later, her creatinine rises to 3.5 mg/dL, and her urine output remains low despite a fluid bolus. A new set of labs are drawn.

Your Challenge:

1. What is the most likely diagnosis for this progression of her kidney injury?
2. What would you expect her FENa and urine osmolality to be now, and what classic finding might you see on urinalysis?

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✅ Answer:

1. The diagnosis is now Acute Tubular Necrosis (ATN), a form of intrinsic AKI. The prolonged hypoperfusion has caused ischemic injury to the tubular cells.
2. With tubular damage, the kidneys can no longer effectively reabsorb sodium or concentrate the urine. Therefore, you would expect FENa > 2% and a urine osmolality < 350 mOsm/kg. Urinalysis would classically show muddy brown granular casts.

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