A 60-year-old man has had angina on exertion for the past 6 years. A coronary angiogram performed 2 years ago showed 75% stenosis of the left circumflex coronary artery and 50% stenosis of the right coronary artery. For the past 3 weeks, the frequency and severity of his anginal attacks have increased, and pain sometimes occurs even when he is lying in bed. On physical examination, his blood pressure is 110/80 mm Hg, and pulse is 85/min with irregular beats. An ECG shows ST-segment elevation. Laboratory studies show serum glucose, 188 mg/dL; creatinine, 1.2 mg/dL; and troponin I, 1.5 ng/mL. Which of the following is most likely to explain these findings?