Review Question - QID 214939

A 43-year-old man presents to his primary care physician concerned about an episode of syncope that occurred 1 week prior while on his daily jog. The patient states that he was halfway through running his usual route when he started feeling a little dizzy and suddenly woke up on the ground. According to witnesses, he lost consciousness for about 10 seconds. Upon further questioning, he has noticed that lately he cannot run as far as he usually does without having to take a break. Family history is significant for multiple heart attacks in several relatives on his father’s side and diabetes on his mother’s side. The patient is otherwise healthy and does not take any medications other than his multivitamins. His current temperature is 98.5°F (36.9°C), blood pressure is 128/78 mmHg, pulse is 65/min, and respirations are 12/min. His physical exam is notable for a grade IV/VI crescendo-decrescendo systolic murmur that radiates to the carotids. The patient is then scheduled for an echocardiogram. What is the most likely etiology of his symptoms?