A 45-year-old man on long-term lithium therapy presents with polyuria and polydipsia. Urine osmolality fails to rise after water deprivation but rises after desmopressin administration. Lithium cannot be stopped. Which drug is most appropriate?

• A Amiloride ✓
• B Hydrochlorothiazide alone
• C Desmopressin long term
• D Furosemide

Explanation

Lithium enters principal cells through ENaC sodium channels and impairs vasopressin signalling, causing nephrogenic diabetes insipidus. Amiloride blocks ENaC, preventing lithium entry into the cell, and is the specific therapy when lithium must be continued. Desmopressin does not work because the collecting duct already cannot respond to vasopressin, which the desmopressin test confirms. B thiazide works only partially through volume contraction.

Reference: Katzung Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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