After a supervised water deprivation test, a patient's urine osmolality plateaus at 180 mOsm/kg despite plasma osmolality of 305 mOsm/kg. Two hours after subcutaneous desmopressin, urine osmolality rises to 520 mOsm/kg. What is the diagnosis?
- A Nephrogenic diabetes insipidus
- B Primary polydipsia
- C Central diabetes insipidus ✓
- D Syndrome of inappropriate antidiuretic hormone secretion
Correct answer: C. Central diabetes insipidus
Explanation
Failure to concentrate urine during dehydration with a marked rise in urine osmolality (more than 50%, often far greater) after exogenous vasopressin indicates deficient endogenous ADH, that is central diabetes insipidus. In nephrogenic disease the kidney cannot respond, so desmopressin produces little change. In primary polydipsia the kidney concentrates normally once plasma osmolality rises. SIADH presents with hyponatraemia, not polyuria.
Reference: Williams Textbook of Endocrinology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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