A 19-year-old man passes 7 litres of dilute urine daily. During a supervised water deprivation test, plasma osmolality rises to 305 mOsm/kg while urine osmolality remains below 300 mOsm/kg and urine output does not fall. After subcutaneous desmopressin, urine osmolality rises to 720 mOsm/kg. What is the diagnosis?
- A Nephrogenic diabetes insipidus
- B Primary polydipsia
- C Central diabetes insipidus ✓
- D Syndrome of inappropriate antidiuresis
Correct answer: C. Central diabetes insipidus
Explanation
Failure to concentrate urine despite maximal dehydration proves true diabetes insipidus rather than primary polydipsia. A rise in urine osmolality of more than 50% after desmopressin shows intact renal tubular responsiveness, localising the defect to deficient pituitary vasopressin secretion, that is, central diabetes insipidus. In nephrogenic disease the kidney cannot respond, so urine osmolality would not rise after desmopressin.
Reference: Williams Textbook of Endocrinology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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