During a water deprivation test, a 26-year-old man with polyuria and polydipsia reaches a plasma sodium of 148 mEq/L with urine osmolality plateauing at 180 mOsm/kg. Desmopressin is given subcutaneously and urine osmolality rises to 480 mOsm/kg within two hours. The diagnosis is:

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

Explanation

In central DI, the kidney responds normally to vasopressin, so exogenous desmopressin produces a large rise in urine osmolality (typically more than 50 percent above baseline). In nephrogenic DI there is no meaningful rise because the renal V2 receptor or aquaporin axis is defective. In primary polydipsia, urine concentrates adequately during deprivation itself and desmopressin adds little further rise. SIADH causes hyponatraemia, not this picture.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.