Central Diabetes Insipidus — RACP Adult Medicine MCQ

Instant feedback + full explanation. One question, done properly.

ModerateEndocrinology & DiabetesCentral Diabetes InsipidusRACP Adult Medicine

A 35-year-old man presents with polyuria, polydipsia, and a serum sodium of 150 mmol/L. His urine is dilute (osmolality 120 mOsm/kg). Water deprivation test shows urine osmolality fails to concentrate above 300 mOsm/kg. After DDAVP administration, urine osmolality rises to 650 mOsm/kg. What is the diagnosis?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: B — Central diabetes insipidus

The water deprivation test shows failure to concentrate urine, confirming diabetes insipidus. The response to DDAVP (significant rise in urine osmolality >50%) confirms central (cranial) DI – the kidneys respond normally to exogenous ADH but there is insufficient endogenous ADH. In nephrogenic DI, DDAVP would have no effect. Causes of central DI include pituitary surgery, craniopharyngioma, Langerhans cell histiocytosis, sarcoidosis, and idiopathic. MRI pituitary should be performed (absence of posterior pituitary bright spot on T1).

Reference: eTG – 2025 – Endocrinology; Endocrine Society – 2023 – Diabetes Insipidus Guidelines