A 30-year-old woman develops polydipsia and polyuria with urine output of 6 L/day after resection of a craniopharyngioma. Plasma osmolality is 302 mOsm/kg, urine osmolality is 85 mOsm/kg. After 8 hours of water deprivation the urine osmolality rises to only 102 mOsm/kg. After desmopressin injection urine osmolality rises to 580 mOsm/kg. The diagnosis is:
- A Central (neurogenic) diabetes insipidus ✓
- B Primary (psychogenic) polydipsia
- C Nephrogenic diabetes insipidus
- D Osmotic diuresis from hyperglycemia
Correct answer: A. Central (neurogenic) diabetes insipidus
Explanation
The water deprivation test distinguishes types of DI. In central DI, the kidneys cannot concentrate urine despite dehydration (urine osmolality remains low), but they respond to exogenous desmopressin with a >50% rise in urine osmolality (here, from 102 to 580 mOsm/kg — a >400% increase). In nephrogenic DI, desmopressin produces <50% rise. In primary polydipsia, water deprivation alone raises urine osmolality adequately. Post-craniopharyngioma surgery is a classic cause of central DI.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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