A 78-year-old man comes to the cardiology clinic because of a 6-month history of chest pressure that occurs when he climbs stairs or walks uphill. The pressure is located in the center of his chest and resolves with rest after 2-3 minutes. Two weeks ago, he felt lightheaded and had to sit down while carrying groceries up to his third-floor apartment. He also reports that he becomes short of breath more easily than he did a year ago. He has hypertension treated with amlodipine. He does not smoke and drinks alcohol occasionally. His temperature is 37.0°C (98.6°F), pulse is 76/min and regular, respirations are 14/min, and blood pressure is 135/85 mm Hg. Pulse oximetry on room air shows an oxygen saturation of 97%. Examination of the carotid arteries shows a pulse that rises slowly and feels diminished in strength bilaterally. Cardiac examination shows a harsh, crescendo-decrescendo systolic murmur, grade 3/6, heard best at the right upper sternal border and radiating to the carotid arteries. S1 is normal, and S2 is single and soft. The lungs are clear to auscultation bilaterally. There is no peripheral edema.
Which of the following is the most likely diagnosis?