A 65-year-old man with a 2-year history of angina pectoris is admitted to the hospital with excruciating substernal chest pain that is not relieved by rest or medication. Physical examination shows diaphoresis and dyspnea. Results of laboratory studies include WBC of 13,000/ μ L, CK-MB of 6.8 ng/mL, and troponin-I of 3.0 ng/mL. An ECG shows ST segment elevation. The patient expires 1 hour after admission. At autopsy, the heart is found to be enlarged but otherwise anatomically normal. Which of the following is the most likely cause of death?