Nephrology and electrolytes: pharmacological aquaresis in SIADH: practice question

A General Medicine 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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General Medicine · Nephrology and electrolytes: pharmacological aquaresis in SIADH

Sample item

A 65-year-old woman with SIADH from small cell lung cancer has serum sodium 123 mmol/L, remains euvolaemic and has not responded to fluid restriction. Which drug class directly opposes the hormone action responsible at the collecting duct?

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

The answer and the reasoning

D. Vasopressin V2 receptor antagonist

In SIADH, excess vasopressin acts on V2 receptors to insert aquaporin-2 channels and retain water. V2 antagonists such as tolvaptan block this and cause free water excretion, though sodium must be monitored to avoid overly rapid correction. Thiazides can worsen hyponatraemia.

Option by option

  • A. Thiazide diuretic

    Thiazides impair dilution and can aggravate hyponatraemia.

  • B. Mineralocorticoid receptor antagonist

    Aldosterone antagonists act on sodium and potassium, not the water channel.

  • C. Selective V1 receptor agonist

    A V1 agonist causes vasoconstriction and does not reduce water retention.

  • D. Vasopressin V2 receptor antagonist (correct)

    Correct. Blocking the V2 receptor produces aquaresis and raises serum sodium.

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 General Medicine question

What is the answer to this General Medicine question?

The best answer is D, Vasopressin V2 receptor antagonist. In SIADH, excess vasopressin acts on V2 receptors to insert aquaporin-2 channels and retain water.

Why is "Thiazide diuretic" not the answer?

Thiazides impair dilution and can aggravate hyponatraemia.

Why is "Mineralocorticoid receptor antagonist" not the answer?

Aldosterone antagonists act on sodium and potassium, not the water channel.

Why is "Selective V1 receptor agonist" not the answer?

A V1 agonist causes vasoconstriction and does not reduce water retention.

What concept does this question test?

It tests Nephrology and electrolytes: pharmacological aquaresis in SIADH in General Medicine. V2 antagonists such as tolvaptan block this and cause free water excretion, though sodium must be monitored to avoid overly rapid correction. Thiazides can worsen hyponatraemia.

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 Nephrology and electrolytes: pharmacological aquaresis in SIADH?

For each disease know the classic presentation, the best test, and the first-line treatment. Then use the flashcards and the other General Medicine 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.