During a water deprivation test, a marked rise in urine osmolality suggests which diagnosis?

• A. Nephrogenic DI
• B. SIADH
• C. Primary polydipsia
• D. Central DI

Answer: (C)

A marked rise in urine osmolality during a water deprivation test shows the kidneys can concentrate urine when ADH is appropriately released, which is typical of primary polydipsia. In primary polydipsia, excessive fluid intake suppresses ADH and yields dilute urine at baseline. When you withhold water, ADH secretion increases, and the kidneys respond by concentrating urine, causing a sharp rise in osmolality.

This contrasts with diabetes insipidus, where the system to concentrate urine is impaired. In central DI, insufficient ADH means urine remains dilute despite dehydration; in nephrogenic DI, the kidneys don’t respond to ADH, so urine stays dilute even with dehydration. SIADH, on the other hand, features inappropriately high ADH with already concentrated urine, so there isn’t the characteristic rapid rise in osmolality with water deprivation.