Electrolytes: nephrogenic diabetes insipidus: 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 · Electrolytes: nephrogenic diabetes insipidus

Sample item

A 40-year-old man with bipolar disorder on long-term lithium passes 6 litres of urine daily. After water deprivation, urine osmolality stays at 160 mosm/kg. After intranasal desmopressin it rises only to 170 mosm/kg. What is the diagnosis and the appropriate step?

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

The answer and the reasoning

A. Nephrogenic diabetes insipidus; review lithium and give a thiazide

Failure to concentrate urine after both dehydration and desmopressin indicates renal resistance to ADH. Lithium is the commonest drug cause, acting by reducing aquaporin-2 expression in collecting duct cells. Management is to stop or lower lithium if possible, and a thiazide with amiloride can reduce urine volume by causing mild volume depletion.

Option by option

  • A. Nephrogenic diabetes insipidus; review lithium and give a thiazide (correct)

    Correct. No response to desmopressin means the kidney is resistant to ADH, and lithium is the likely cause.

  • B. Primary polydipsia; fluid restriction

    In primary polydipsia urine concentrates normally during water deprivation.

  • C. SIADH; start tolvaptan

    SIADH produces concentrated urine and hyponatraemia, not 6 litres of dilute urine daily.

  • D. Central diabetes insipidus; start desmopressin

    Central diabetes insipidus would show a rise of more than 50% in urine osmolality after desmopressin.

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 A, Nephrogenic diabetes insipidus; review lithium and give a thiazide. Failure to concentrate urine after both dehydration and desmopressin indicates renal resistance to ADH.

Why is "Primary polydipsia; fluid restriction" not the answer?

In primary polydipsia urine concentrates normally during water deprivation.

Why is "SIADH; start tolvaptan" not the answer?

SIADH produces concentrated urine and hyponatraemia, not 6 litres of dilute urine daily.

Why is "Central diabetes insipidus; start desmopressin" not the answer?

Central diabetes insipidus would show a rise of more than 50% in urine osmolality after desmopressin.

What concept does this question test?

It tests Electrolytes: nephrogenic diabetes insipidus in General Medicine. Lithium is the commonest drug cause, acting by reducing aquaporin-2 expression in collecting duct cells. Management is to stop or lower lithium if possible, and a thiazide with amiloride can reduce urine volume by causing mild volume depletion.

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 Electrolytes: nephrogenic diabetes insipidus?

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.