Renal and acid–base: urinary indices in prerenal azotaemia: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · Renal and acid–base: urinary indices in prerenal azotaemia

Sample item

A 30-year-old man has had severe vomiting and diarrhoea for 3 days. His serum creatinine is raised, but his urine sediment is bland. Which urinary finding favours a prerenal cause over acute tubular necrosis?

ALow urinary sodium with high urine osmolalityBHigh urinary sodium with a high fractional excretion of sodiumCUrine osmolality close to that of plasmaDMuddy brown granular casts

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Low urinary sodium with high urine osmolality

In prerenal azotaemia the tubules are intact and respond to volume depletion with strong sodium and water conservation through the renin-angiotensin-aldosterone system and ADH. The urine is therefore concentrated with a low sodium content. In acute tubular necrosis tubular function is lost, so urine is isosmotic with high sodium.

Option by option

A. Low urinary sodium with high urine osmolality (correct)
Correct. Intact tubules avidly retain sodium and water.
B. High urinary sodium with a high fractional excretion of sodium
A high urinary sodium and high fractional excretion indicate tubular injury, not a prerenal state.
C. Urine osmolality close to that of plasma
An isosmotic urine indicates loss of concentrating ability, as seen in tubular necrosis.
D. Muddy brown granular casts
Muddy brown granular casts are the sediment finding of acute tubular necrosis.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Low urinary sodium with high urine osmolality. In prerenal azotaemia the tubules are intact and respond to volume depletion with strong sodium and water conservation through the renin-angiotensin-aldosterone system and ADH.

Why is "High urinary sodium with a high fractional excretion of sodium" not the answer?

A high urinary sodium and high fractional excretion indicate tubular injury, not a prerenal state.

Why is "Urine osmolality close to that of plasma" not the answer?

An isosmotic urine indicates loss of concentrating ability, as seen in tubular necrosis.

Why is "Muddy brown granular casts" not the answer?

Muddy brown granular casts are the sediment finding of acute tubular necrosis.

What concept does this question test?

It tests Renal and acid–base: urinary indices in prerenal azotaemia in Physiology. The urine is therefore concentrated with a low sodium content. In acute tubular necrosis tubular function is lost, so urine is isosmotic with high sodium.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Renal and acid–base: urinary indices in prerenal azotaemia?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.