A patient undergoes a prolonged water deprivation test followed by parenteral desmopressin. Baseline urine osmolality remains below 300 mOsm/kg after dehydration. After desmopressin, urine osmolality rises to 620 mOsm/kg. Which diagnosis fits this response?

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

Explanation

C rise in urine osmolality exceeding roughly 50 percent after desmopressin indicates deficient endogenous ADH that responds to exogenous analogue, defining central diabetes insipidus. In nephrogenic diabetes insipidus the collecting duct cannot respond, so the increase stays below about 10 percent. Primary polydipsia shows adequate concentration during dehydration itself before any drug is given. SIADH causes concentrated urine at baseline rather than dilute polyuria, eliminating option D.

Reference: Harrison Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.