MSFinals-4801

An 82-year-old man is admitted to the emergency department with symptoms of confusion, lethargy and repeated vomiting. He has a medical history of heart failure, hypertension and atrial fibrillation, which is managed with digoxin. During a recent medication review with his general practitioner, he was prescribed a new medication. Upon examination, his heart rate is 34/min, respiratory rate 15/min, blood pressure 90/65 mmHg and temperature 35.9 ºC. An electrocardiogram reveals downsloping ST depression and inverted T waves. Which medication is most likely responsible for exacerbating his symptoms?