A 32-year-old man has polyuria of 7 L/day and serum sodium of 149 mEq/L. During a water deprivation test his urine osmolality plateaus at 180 mOsm/kg. After subcutaneous desmopressin, urine osmolality rises to 520 mOsm/kg. Which is the most likely diagnosis?

• A Primary polydipsia
• B Nephrogenic diabetes insipidus
• C Central diabetes insipidus ✓
• D Osmotic diuresis from undiagnosed diabetes mellitus

Explanation

Inability to concentrate urine above plasma osmolality during water deprivation shows true diabetes insipidus. A rise of more than 50 percent in urine osmolality after desmopressin indicates intact renal V2 receptor function, which defines central DI. In nephrogenic DI the kidney cannot respond, so osmolality stays low. Primary polydipsia concentrates urine normally during deprivation, and glycosuria is excluded by the clinical picture.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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