Renal: urinary concentration and urea: 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: urinary concentration and urea

Sample item

A child with severe protein-energy malnutrition cannot concentrate urine maximally even though the ADH level is high and the collecting duct responds to ADH. Which mechanism most likely accounts for this?

AA mutated V2 receptor in the collecting ductBFailure of the Na-K-2Cl cotransporter in the proximal tubuleCA markedly raised GFR washing out the medullaDLow urea supply reduces urea recycling and the inner medullary osmotic gradient

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

The answer and the reasoning

D. Low urea supply reduces urea recycling and the inner medullary osmotic gradient

About half of the inner medullary interstitial osmolality is contributed by urea. ADH increases urea permeability of the inner medullary collecting duct through UT-A1, so urea recycles into the medulla. With protein deficiency, urea production is low, the medullary gradient is blunted and maximal urine osmolality falls.

Option by option

A. A mutated V2 receptor in the collecting duct
The stem states that the collecting duct responds to ADH, which excludes a receptor defect.
B. Failure of the Na-K-2Cl cotransporter in the proximal tubule
The Na-K-2Cl cotransporter is in the thick ascending limb, not the proximal tubule.
C. A markedly raised GFR washing out the medulla
GFR is not raised in malnutrition; it is usually reduced.
D. Low urea supply reduces urea recycling and the inner medullary osmotic gradient (correct)
Correct. Reduced urea recycling lowers medullary osmolality and concentrating ability.

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 D, Low urea supply reduces urea recycling and the inner medullary osmotic gradient. About half of the inner medullary interstitial osmolality is contributed by urea.

Why is "A mutated V2 receptor in the collecting duct" not the answer?

The stem states that the collecting duct responds to ADH, which excludes a receptor defect.

Why is "Failure of the Na-K-2Cl cotransporter in the proximal tubule" not the answer?

The Na-K-2Cl cotransporter is in the thick ascending limb, not the proximal tubule.

Why is "A markedly raised GFR washing out the medulla" not the answer?

GFR is not raised in malnutrition; it is usually reduced.

What concept does this question test?

It tests Renal: urinary concentration and urea in Physiology. ADH increases urea permeability of the inner medullary collecting duct through UT-A1, so urea recycles into the medulla. With protein deficiency, urea production is low, the medullary gradient is blunted and maximal urine osmolality falls.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. 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: urinary concentration and urea?

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.