Renal and acid–base: primary polydipsia: 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: primary polydipsia

Sample item

A 35-year-old woman with schizophrenia drinks about 10 litres of water daily. Her serum sodium is 124 mmol/L and urine osmolality is 60 mOsm/kg. What is the best explanation for the hyponatraemia?

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

The answer and the reasoning

C. Appropriately suppressed ADH, with water intake exceeding the kidney's capacity to excrete it

The kidney can excrete a very dilute urine, with an osmolality as low as 50 mOsm/kg, but its maximum free water excretion is limited. Drinking beyond this limit dilutes the plasma. ADH is appropriately suppressed, so the urine is maximally dilute, unlike SIADH, where the urine is inappropriately concentrated.

Option by option

  • A. Inappropriate secretion of ADH from a tumour

    Inappropriate ADH would give a concentrated urine, not an osmolality of 60.

  • B. Nephrogenic resistance to ADH in the collecting duct

    Nephrogenic resistance to ADH would give polyuria with hypernatraemia rather than hyponatraemia.

  • C. Appropriately suppressed ADH, with water intake exceeding the kidney's capacity to excrete it (correct)

    Correct. A maximally dilute urine shows that ADH is suppressed, and intake simply exceeds output capacity.

  • D. Mineralocorticoid deficiency with sodium wasting

    Mineralocorticoid deficiency would cause volume depletion and a concentrated urine.

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 C, Appropriately suppressed ADH, with water intake exceeding the kidney's capacity to excrete it. The kidney can excrete a very dilute urine, with an osmolality as low as 50 mOsm/kg, but its maximum free water excretion is limited.

Why is "Inappropriate secretion of ADH from a tumour" not the answer?

Inappropriate ADH would give a concentrated urine, not an osmolality of 60.

Why is "Nephrogenic resistance to ADH in the collecting duct" not the answer?

Nephrogenic resistance to ADH would give polyuria with hypernatraemia rather than hyponatraemia.

Why is "Mineralocorticoid deficiency with sodium wasting" not the answer?

Mineralocorticoid deficiency would cause volume depletion and a concentrated urine.

What concept does this question test?

It tests Renal and acid–base: primary polydipsia in Physiology. Drinking beyond this limit dilutes the plasma. ADH is appropriately suppressed, so the urine is maximally dilute, unlike SIADH, where the urine is inappropriately concentrated.

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: primary polydipsia?

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.