After overnight water deprivation, a patient has plasma osmolality of 305 mOsm/kg and urine osmolality of 110 mOsm/kg. Desmopressin is given and repeat urine osmolality is 115 mOsm/kg. These findings indicate:

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

Explanation

Water deprivation concentrates plasma but fails to produce concentrated urine, excluding primary polydipsia. After exogenous desmopressin, urine osmolality should rise substantially in central diabetes insipidus because the collecting ducts retain functional vasopressin V2 receptors. A rise of less than 50 percent, here essentially nil, shows the renal tubules cannot respond, establishing nephrogenic diabetes insipidus. Complete central disease would respond fully to desmopressin, and partial central disease responds at least partially.

Reference: Williams Textbook of Endocrinology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.