Clinical biochemistry: water deprivation test: practice question

A Biochemistry 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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Biochemistry · Clinical biochemistry: water deprivation test

Sample item

A man passes 8 litres of dilute urine daily. After water deprivation, urine osmolality stays at 150 mOsm/kg, and after desmopressin it rises to 600 mOsm/kg. What is the diagnosis?

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

The answer and the reasoning

B. Central diabetes insipidus

Failure to concentrate urine during water deprivation, with a prompt rise after desmopressin, shows that the kidney can respond but vasopressin is lacking: central diabetes insipidus. In nephrogenic disease there is no rise after desmopressin. In primary polydipsia, urine concentrates during water deprivation.

Option by option

  • A. Nephrogenic diabetes insipidus

    Nephrogenic diabetes insipidus would show little rise in osmolality after desmopressin.

  • B. Central diabetes insipidus (correct)

    Correct. A large rise after desmopressin shows the kidney is responsive and vasopressin is deficient.

  • C. Primary polydipsia

    Primary polydipsia would concentrate urine during water deprivation.

  • D. Osmotic diuresis from glycosuria

    Osmotic diuresis from glycosuria gives a urine of high or intermediate osmolality and positive urine glucose.

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 Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Central diabetes insipidus. Failure to concentrate urine during water deprivation, with a prompt rise after desmopressin, shows that the kidney can respond but vasopressin is lacking: central diabetes insipidus.

Why is "Nephrogenic diabetes insipidus" not the answer?

Nephrogenic diabetes insipidus would show little rise in osmolality after desmopressin.

Why is "Primary polydipsia" not the answer?

Primary polydipsia would concentrate urine during water deprivation.

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

Osmotic diuresis from glycosuria gives a urine of high or intermediate osmolality and positive urine glucose.

What concept does this question test?

It tests Clinical biochemistry: water deprivation test in Biochemistry. In nephrogenic disease there is no rise after desmopressin. In primary polydipsia, urine concentrates during water deprivation.

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 Clinical biochemistry: water deprivation test?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry 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.