Review Question - QID 214437

A 27-year-old man presents to his primary care physician for a routine examination. He currently has no acute complaints and says he is otherwise healthy. At times he experiences palpitations and anxiety. He exercises 5 times a week and eats a generally healthy diet. He takes a daily multivitamin and supplemental protein powder. He previously had "strep throat" when he was 11-years-old and was treated with oral penicillin. His father has a history of aortic stenosis that required valve replacement. His mother is otherwise healthy. Physical examination is remarkable for a 3/6 holosystolic murmur that is best heard in the left mid-to-lower sternal border. There is an associated thrill. Which of the following is the most likely diagnosis?