Renal: water excretion and ADH excess: 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: water excretion and ADH excess

Sample item

A 62-year-old smoker with small cell lung cancer is euvolaemic with serum sodium 118 mmol/L, serum osmolality 250 mOsm/kg, urine osmolality 480 mOsm/kg and urine sodium 55 mmol/L. What explains his hyponatraemia?

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

The answer and the reasoning

B. Inappropriate ADH-mediated water retention through aquaporin-2 insertion

Ectopic ADH from the tumour raises collecting duct water permeability despite low plasma osmolality, so urine is inappropriately concentrated. Retained water dilutes plasma sodium, and natriuresis follows from volume expansion. Fluid restriction, or an ADH antagonist, is the physiological remedy.

Option by option

  • A. Increased water intake with maximally dilute urine

    Primary polydipsia gives urine osmolality below 100, not 480.

  • B. Inappropriate ADH-mediated water retention through aquaporin-2 insertion (correct)

    Correct. Excess ADH retains water so plasma becomes dilute while urine stays concentrated.

  • C. Aldosterone deficiency causing renal sodium wasting

    Aldosterone deficiency would cause hypovolaemia and hyperkalaemia.

  • D. Osmotic diuresis from hyperglycaemia

    Hyperglycaemia causes hypertonic hyponatraemia, but the serum osmolality is low here.

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 B, Inappropriate ADH-mediated water retention through aquaporin-2 insertion. Ectopic ADH from the tumour raises collecting duct water permeability despite low plasma osmolality, so urine is inappropriately concentrated.

Why is "Increased water intake with maximally dilute urine" not the answer?

Primary polydipsia gives urine osmolality below 100, not 480.

Why is "Aldosterone deficiency causing renal sodium wasting" not the answer?

Aldosterone deficiency would cause hypovolaemia and hyperkalaemia.

Why is "Osmotic diuresis from hyperglycaemia" not the answer?

Hyperglycaemia causes hypertonic hyponatraemia, but the serum osmolality is low here.

What concept does this question test?

It tests Renal: water excretion and ADH excess in Physiology. Retained water dilutes plasma sodium, and natriuresis follows from volume expansion. Fluid restriction, or an ADH antagonist, is the physiological remedy.

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: water excretion and ADH excess?

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.