In nephrogenic diabetes insipidus, a desmopressin challenge typically shows which result?

• A. No change in urine osmolality
• B. Marked increase in urine osmolality
• C. Decrease in urine osmolality
• D. Increase in serum osmolality

Answer: (A)

In nephrogenic diabetes insipidus the kidneys do not respond to vasopressin because the defect is at the level of the kidney's receptors or water channels. Desmopressin, a synthetic vasopressin analog, cannot drive water reabsorption in the collecting ducts of someone with this condition. As a result, the urine remains dilute and urine osmolality does not rise after the challenge.

This contrasts with central diabetes insipidus, where there is a deficiency of vasopressin production; giving desmopressin markedly increases urine osmolality by promoting water reabsorption. A decrease in urine osmolality isn’t the expected response to desmopressin in this context, and focusing on serum osmolality isn’t the typical diagnostic readout of the test—the key point is whether urine becomes more concentrated.