A 40-year-old man with focal epilepsy on carbamazepine 600 mg twice daily for 4 weeks presents with nausea, headache, and confusion. Serum sodium is 122 mEq/L. Serum osmolality is low and urine osmolality is inappropriately elevated. What is the most likely cause?
- A Cerebral salt wasting
- B Carbamazepine-induced SIADH ✓
- C Addisonian crisis
- D Excessive free water intake
Correct answer: B. Carbamazepine-induced SIADH
Explanation
Carbazine is a well-known cause of SIADH, presenting with hyponatremia, low serum osmolality, and inappropriately concentrated urine. The temporal association (4 weeks after starting) supports drug-induced SIADH. Cerebral salt wasting is rare and usually follows subarachnoid hemorrhage with volume depletion. Addisonian crisis causes hyperkalemia and hypotension.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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