Which condition is treated with desmopressin (DDAVP)?

• A. Nephrogenic diabetes insipidus
• B. Syndrome of inappropriate antidiuretic hormone
• C. Central diabetes insipidus
• D. Primary polydipsia

Answer: (C)

Desmopressin is used to treat central diabetes insipidus because the problem in this condition is a deficiency of antidiuretic hormone (ADH) from the posterior pituitary. Desmopressin is a synthetic vasopressin analog that acts on the kidneys to increase water reabsorption in the collecting ducts via V2 receptors. This reduces urine output and concentrates the urine, addressing the underlying lack of ADH.

In contrast, nephrogenic diabetes insipidus involves kidneys that don't respond to ADH, so giving desmopressin won't help. In SIADH, there is already excess ADH, and adding more would worsen hyponatremia and fluid retention. Primary polydipsia is due to excessive water intake rather than a deficiency of ADH, so desmopressin isn't appropriate. Central diabetes insipidus best fits the mechanism of action for desmopressin, making it the correct treatment.