Heart blocks: Pathology review

A 71-year-old man presents to the emergency department with sudden onset chest pain. He was sitting at home watching television when he noticed the pain, characterized as a sharp pressure-like sensation in the left side of his chest. He is having difficulty catching his breath. Medical history is significant for hypertension, COPD, chronic kidney disease, and coronary artery disease. He takes amlodipine, lisinopril, aspirin, salmeterol, and albuterol. His father died from a heart attack at age 50. He smokes one pack of cigarettes per day and drinks occasionally. His temperature is 37.2°C (99°F), pulse is 105/min, respirations are 23/min, blood pressure is 90/60 mmHg, and oxygen saturation is 90% on room air. He appears pale and diaphoretic. Physical exam shows a holosystolic murmur at the left mid sternal border that increases with supine leg raise. Lung examination is normal. An ECG is obtained and shows the following:  



 Reproduced from: Wikimedia Commons

Which of the following is the most likely cause of this patient’s symptoms?