A 27-year-old man presents with polyuria, polydipsia, and hypernatremia (Na+ 152 mEq/L). Water deprivation test fails to concentrate urine above 300 mOsm/kg. Urine osmolality rises to 650 mOsm/kg after exogenous desmopressin. This response is consistent with:

• A Nephrogenic diabetes insipidus
• B Central (neurogenic) diabetes insipidus ✓
• C Primary polydipsia (psychogenic)
• D Partial nephrogenic diabetes insipidus

Explanation

In central DI, the neurohypophysis fails to secrete adequate ADH; the kidney's V2 receptors are intact, so exogenous desmopressin produces a marked rise in urine osmolality (>50% increase, often reaching >750 mOsm/kg). Nephrogenic DI shows minimal or no response to desmopressin because renal V2 receptors are defective. Primary polydipsia produces dilute urine with water deprivation but retains some concentrating ability as renal medullary gradient is partially washed out. The robust desmopressin response to >650 mOsm/kg here distinguishes complete central DI.

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

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Written and medically reviewed by the StethoPrep medical team.