Renal and acid–base: osmotic diuresis with mannitol: 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: osmotic diuresis with mannitol

Sample item

A patient with raised intracranial pressure is given intravenous mannitol and urine volume rises sharply. What is the renal mechanism?

AIt inhibits carbonic anhydrase in the proximal tubuleBIt is filtered but not reabsorbed, keeping water in the tubular lumenCIt blocks the Na-K-2Cl cotransporter in the thick ascending limbDIt antagonises vasopressin at the V2 receptor

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

The answer and the reasoning

B. It is filtered but not reabsorbed, keeping water in the tubular lumen

Mannitol is freely filtered and not reabsorbed, so it raises tubular fluid osmolality. This retains water in the lumen and washes out sodium and water, particularly in the proximal tubule and loop. It also acts in the plasma as an osmotic agent that pulls water out of brain tissue.

Option by option

A. It inhibits carbonic anhydrase in the proximal tubule
This is the action of acetazolamide.
B. It is filtered but not reabsorbed, keeping water in the tubular lumen (correct)
Correct. The unreabsorbed solute holds water in the lumen, giving an osmotic diuresis.
C. It blocks the Na-K-2Cl cotransporter in the thick ascending limb
This is the action of loop diuretics such as furosemide.
D. It antagonises vasopressin at the V2 receptor
This is the action of the vaptans, for example tolvaptan.

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 B, It is filtered but not reabsorbed, keeping water in the tubular lumen. Mannitol is freely filtered and not reabsorbed, so it raises tubular fluid osmolality.

Why is "It inhibits carbonic anhydrase in the proximal tubule" not the answer?

This is the action of acetazolamide.

Why is "It blocks the Na-K-2Cl cotransporter in the thick ascending limb" not the answer?

This is the action of loop diuretics such as furosemide.

Why is "It antagonises vasopressin at the V2 receptor" not the answer?

This is the action of the vaptans, for example tolvaptan.

What concept does this question test?

It tests Renal and acid–base: osmotic diuresis with mannitol in Physiology. This retains water in the lumen and washes out sodium and water, particularly in the proximal tubule and loop. It also acts in the plasma as an osmotic agent that pulls water out of brain tissue.

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: osmotic diuresis with mannitol?

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.