A 30-year-old woman undergoes a water deprivation test for polyuria and polydipsia. After 6 hours of deprivation her urine osmolality is 120 mOsm/kg and plasma osmolality is 298 mOsm/kg. After desmopressin administration, urine osmolality remains 125 mOsm/kg. These findings indicate:

• A Complete central diabetes insipidus
• B Partial central diabetes insipidus
• C Primary polydipsia
• D Nephrogenic diabetes insipidus ✓

Explanation

Failure to concentrate urine above plasma osmolality after prolonged dehydration proves an inability to respond to ADH, and the absence of any rise after exogenous desmopressin localises the defect to the renal collecting duct vasopressin V2 receptor or aquaporin-2 pathway. In central diabetes insipidus, desmopressin would raise urine osmolality substantially, often by more than 50%. In primary polydipsia, urine concentrates well during deprivation itself.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.