A 62-year-old woman with a history of hypertension and dyslipidemia presents with a 2-month history of exertional chest discomfort. Stress ECG shows ST-segment depression in leads II, III, and aVF, consistent with inferior wall ischemia. Coronary angiography reveals 70% stenosis of the right coronary artery. She is started on dual antiplatelet therapy, a statin, and an ACE inhibitor. For antianginal therapy, she is prescribed diltiazem (a non-dihydropyridine calcium channel blocker) 120 mg twice daily. After 1 week, she reports excellent symptom relief. However, she develops fatigue, dyspnea on mild exertion, and a persistent dry cough. On examination, heart rate is 48/min, blood pressure is 110/68 mmHg, and lung auscultation reveals bibasal crackles. Echocardiography shows an ejection fraction of 35% (previously 50%). What is the most likely explanation for her clinical deterioration?