Posterior pituitary: SIADH: 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 · Posterior pituitary: SIADH

Sample item

A 62-year-old smoker with small cell lung cancer is euvolaemic with serum sodium 122 mmol/L, serum osmolality 255 mOsm/kg, urine osmolality 450 mOsm/kg and urine sodium 48 mmol/L. What is the best explanation?

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

The answer and the reasoning

A. Inappropriate ADH secretion causing water retention and dilutional hyponatraemia

Ectopic ADH release by tumour cells causes renal water retention in spite of low plasma osmolality. Dilutional hyponatraemia with an inappropriately concentrated urine, urinary sodium above 40 mmol/L and euvolaemia is diagnostic of SIADH. Thyroid, adrenal and renal causes must be excluded first.

Option by option

  • A. Inappropriate ADH secretion causing water retention and dilutional hyponatraemia (correct)

    Correct. A concentrated urine with low serum osmolality shows inappropriate ADH action.

  • B. Central diabetes insipidus

    Central diabetes insipidus causes dilute urine and a high or normal serum sodium.

  • C. Extrarenal sodium loss from diarrhoea

    Diarrhoea causes hypovolaemia with a low urine sodium.

  • D. Primary polydipsia with suppressed ADH

    Primary polydipsia gives maximally dilute urine, with osmolality below 100.

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, Inappropriate ADH secretion causing water retention and dilutional hyponatraemia. Ectopic ADH release by tumour cells causes renal water retention in spite of low plasma osmolality.

Why is "Central diabetes insipidus" not the answer?

Central diabetes insipidus causes dilute urine and a high or normal serum sodium.

Why is "Extrarenal sodium loss from diarrhoea" not the answer?

Diarrhoea causes hypovolaemia with a low urine sodium.

Why is "Primary polydipsia with suppressed ADH" not the answer?

Primary polydipsia gives maximally dilute urine, with osmolality below 100.

What concept does this question test?

It tests Posterior pituitary: SIADH in Physiology. Dilutional hyponatraemia with an inappropriately concentrated urine, urinary sodium above 40 mmol/L and euvolaemia is diagnostic of SIADH. Thyroid, adrenal and renal causes must be excluded first.

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 Posterior pituitary: SIADH?

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.