A 70-year-old male with a 10-year history of diabetes mellitus (DM), previously no history of palpitations or chest pain. One hour after breakfast today, he suddenly developed significant chest tightness, pallor, restlessness, sweating, and a sense of fear. Symptoms persisted for 2 hours before he presented to the emergency department. Physical examination: heart rate 100 bpm, blood pressure 86/70 mmHg. The most likely diagnosis is: