An 18-year-old man is admitted to the hospital with acute onset of substernal chest pain that began abruptly 30 minutes ago. He repos the pain radiating to his neck and right arm. He has otherwise been in good health. On physical examination, he is diaphoretic and tachypnoeic. His BP is 102/48 mmHg and hea rate is 112 bpm, regular rhythm but is tachycardiac. A 2/6 holosystolic murmur is heard best at the apex and radiates to the axilla. His lungs have bilateral rales at the bases. The ECG demonstrates 4 mm of ST elevation in the anterior leads. In the past he was hospitalized for some problem with his hea when he was 4 years old. His mother, who accompanies him, repos that he received aspirin and g- globulin as treatment. Since that time, he has required intermittent follow-up with echocardiography. What is the most likely cause of this patient's acute coronary syndrome?