What's wrong with this artery? A medical disease discovered by a surgeon.

A 50-year-old man presenting with chest pain had positive stress echocardiography; and angiogram showed single artery coronary stenosis, presumed to be atherosclerotic. He was started on optimal medical therapy with good compliance. Four months later, he had a myocardial infarction (MI) and cardiac catheterisation surprisingly showed interval development of severe three-vessel stenosis.