A 65-year-old woman with a history of epilepsy has been taking carbamazepine for the past 6 months. She presents to the clinic with complaints of fatigue, nausea, and confusion. Laboratory tests reveal hyponatremia (serum sodium: 125 mEq/L), low plasma osmolality, and inappropriately elevated urine osmolality. Which of the following is the most likely cause of her symptoms?