After desmopressin administration, which pattern distinguishes central diabetes insipidus from nephrogenic DI?

• A. No change in urine osmolality
• B. Decrease in urine osmolality
• C. Increase in urine osmolality
• D. Rapid drop in serum osmolality

Answer: (C)

Desmopressin is a synthetic ADH analog used to see how the kidneys respond to ADH. In central diabetes insipidus, there is a deficiency of endogenous ADH, so giving desmopressin allows the collecting ducts to reabsorb more water. This leads to more concentrated urine, reflected by a rise in urine osmolality. In contrast, nephrogenic diabetes insipidus features kidney insensitivity to ADH, so desmopressin does not substantially change the urine’s concentration, and urine osmolality stays low. Therefore, the distinguishing pattern is an increase in urine osmolality after desmopressin administration.