A 25-year-old man presents with polyuria and polydipsia. After controlled water deprivation, his urine osmolality remains low at 130 mOsm/kg. One hour after administration of desmopressin, urine osmolality rises to 480 mOsm/kg. Which diagnosis best explains these findings?

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

Explanation

Central diabetes insipidus results from deficient ADH secretion, so the kidney retains the capacity to respond when exogenous desmopressin supplies the missing hormone, producing a marked rise in urine osmolality (typically more than 50 percent increase). In nephrogenic DI the collecting ducts cannot respond to ADH, so desmopressin produces no significant rise. In primary polydipsia, urine concentrates appropriately during water deprivation itself, so the deprivation test alone discriminates it.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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