Water balance: water deprivation test: 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 · Water balance: water deprivation test

Sample item

A 30-year-old man with a recent head injury passes large volumes of dilute urine. After water deprivation his urine remains dilute, but after desmopressin it becomes markedly concentrated. What is the diagnosis?

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

The answer and the reasoning

C. Central diabetes insipidus

A rise in urine osmolality after desmopressin shows that the collecting duct can respond to ADH, so the defect is deficient ADH release, as after pituitary or hypothalamic injury. In nephrogenic disease the kidney does not respond, and in primary polydipsia the kidney concentrates urine when water is withheld.

Option by option

  • A. Primary polydipsia

    In primary polydipsia ADH release is intact, so urine becomes concentrated during water deprivation.

  • B. Nephrogenic diabetes insipidus

    In nephrogenic diabetes insipidus the collecting duct is resistant to ADH, so desmopressin does not concentrate the urine.

  • C. Central diabetes insipidus (correct)

    Correct. The kidney responds to exogenous ADH, so the fault is lack of ADH secretion.

  • D. Syndrome of inappropriate ADH secretion

    SIADH causes concentrated urine and hyponatraemia, not polyuria with dilute 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, Central diabetes insipidus. A rise in urine osmolality after desmopressin shows that the collecting duct can respond to ADH, so the defect is deficient ADH release, as after pituitary or hypothalamic injury.

Why is "Primary polydipsia" not the answer?

In primary polydipsia ADH release is intact, so urine becomes concentrated during water deprivation.

Why is "Nephrogenic diabetes insipidus" not the answer?

In nephrogenic diabetes insipidus the collecting duct is resistant to ADH, so desmopressin does not concentrate the urine.

Why is "Syndrome of inappropriate ADH secretion" not the answer?

SIADH causes concentrated urine and hyponatraemia, not polyuria with dilute urine.

What concept does this question test?

It tests Water balance: water deprivation test in Physiology. In nephrogenic disease the kidney does not respond, and in primary polydipsia the kidney concentrates urine when water is withheld.

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 Water balance: water deprivation test?

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.