A 32-year-old man with a history of transsphenoidal surgery for a craniopharyngioma has polyuria of 9 L/day. During a supervised water deprivation test, urine osmolality plateaus at 180 mOsm/kg after 6 hours while plasma osmolality rises to 305 mOsm/kg. After desmopressin, urine osmolality rises to 520 mOsm/kg. The diagnosis is:

• A Primary polydipsia
• B Nephrogenic diabetes insipidus
• C Syndrome of inappropriate antidiuresis
• D Central diabetes insipidus ✓

Explanation

Inability to concentrate urine despite a raised plasma osmolality establishes true diabetes insipidus. A rise in urine osmolality of more than 50% after desmopressin indicates vasopressin-responsive tubules, which defines central DI. In nephrogenic DI the response to desmopressin is absent because the renal V2 receptor or aquaporin pathway fails. In primary polydipsia the patient concentrates urine normally before desmopressin, and SIADH causes concentrated urine with low plasma osmolality.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.