A 30-year-old man comes to the physician because he feels weak and faint during heavy exercise. This occurred three times during the past month. He has no history of serious illness and takes no medications. There is a family history of sudden death. His pulse is 100/min and regular, respirations are 20/min, and blood pressure is 130/75 mm Hg. Cardiac examination shows a systolic thrill and a systolic ejection murmur heard best over the lower left sternal border. The murmur is louder with the Valsalva maneuver. Which of the following is the most likely diagnosis?