A 40-year-old woman on carbamazepine for trigeminal neuralgia presents with nausea, headache, and confusion. Serum sodium is 122 mEq/L. The most likely mechanism of hyponatremia is:
- A Syndrome of inappropriate antidiuretic hormone secretion (SIADH) ✓
- B Renal tubular acidosis
- C Primary adrenal insufficiency
- D Excessive diuretic use
Correct answer: A. Syndrome of inappropriate antidiuretic hormone secretion (SIADH)
Explanation
Carbamazepine frequently causes hyponatremia by enhancing the action of antidiuretic hormone on the renal collecting duct, leading to SIADH. This effect is more common in the elderly and can present with confusion and seizures. Serum sodium should be monitored during therapy.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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