Renal and acid–base: urea handling and the BUN to creatinine ratio: 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: urea handling and the BUN to creatinine ratio

Sample item

A dehydrated patient has a raised blood urea nitrogen to creatinine ratio, although the GFR has fallen only moderately. Which difference in renal handling explains the disproportionate rise in urea?

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

The answer and the reasoning

A. Urea is reabsorbed in the tubules, more so when flow is slow and ADH is high, whereas creatinine is not reabsorbed

Urea is filtered and then partly reabsorbed passively, in the proximal tubule following water and in the medullary collecting duct under ADH. Slow tubular flow and high ADH increase this reabsorption. Creatinine is filtered and slightly secreted but not reabsorbed, so its level reflects the GFR more directly.

Option by option

  • A. Urea is reabsorbed in the tubules, more so when flow is slow and ADH is high, whereas creatinine is not reabsorbed (correct)

    Correct. Tubular urea reabsorption rises in volume depletion, while creatinine is not reabsorbed.

  • B. Creatinine is reabsorbed passively with water, whereas urea is actively secreted

    Creatinine is not reabsorbed, and urea is not actively secreted.

  • C. Urea is not filtered at the glomerulus, whereas creatinine is freely filtered

    Urea is freely filtered, so its handling begins with filtration, just like creatinine.

  • D. Urea is produced in the kidney in proportion to dehydration

    Urea is made in the liver from protein breakdown, not in the kidney.

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.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Urea is reabsorbed in the tubules, more so when flow is slow and ADH is high, whereas creatinine is not reabsorbed. Urea is filtered and then partly reabsorbed passively, in the proximal tubule following water and in the medullary collecting duct under ADH.

Why is "Creatinine is reabsorbed passively with water, whereas urea is active…" not the answer?

Creatinine is not reabsorbed, and urea is not actively secreted.

Why is "Urea is not filtered at the glomerulus, whereas creatinine is freely…" not the answer?

Urea is freely filtered, so its handling begins with filtration, just like creatinine.

Why is "Urea is produced in the kidney in proportion to dehydration" not the answer?

Urea is made in the liver from protein breakdown, not in the kidney.

What concept does this question test?

It tests Renal and acid–base: urea handling and the BUN to creatinine ratio in Physiology. Slow tubular flow and high ADH increase this reabsorption. Creatinine is filtered and slightly secreted but not reabsorbed, so its level reflects the GFR more directly.

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 and acid–base: urea handling and the BUN to creatinine ratio?

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.