Review Question - QID 216420

A 51-year-old man presents to his primary care physician with a 6-month history of increasing fatigue. He says that he started noticing that he was getting winded while at work, but thought it was simply a result of getting older. He became concerned, however, when he was no longer able to walk to his car without feeling short of breath. He has no significant past medical history and has never traveled outside the United States. His temperature is 98.6°F (37.0°C), blood pressure is 110/50 mmHg, pulse is 70/min, and respirations are 12/min. On physical exam, he is found to have an early diastolic decrescendo murmur best heard at the lower left sternal border. The murmur intensity increases in intensity with hand grip and decreases with forced expiration against a closed glottis. Which of the following is the most likely underlying cause of this patient's symptoms?