Q. A 68-year-old male comes to the clinic with a complaint of increasing fatigue over the past few months, especially during exertion. He also mentions feeling lightheaded occasionally when he stands up. He has no history of chest pain, palpitations, or syncope. His past medical history is significant for hypertension, which has been under control with medication. He denies any smoking, alcohol abuse, or illicit drug use. On physical examination, his blood pressure is 145/80 mmHg, heart rate is 72 beats/min, and he has a regular rhythm. A prominent carotid pulse is palpable, and a harsh crescendo-decrescendo systolic murmur is heard best at the right upper sternal border, which radiates to the neck. There is no jugular venous distension, and his lungs are clear to auscultation. An electrocardiogram (ECG) shows left ventricular hypertrophy, but no signs of ischemia. Chest x-ray reveals a calcified aortic valve with no pulmonary congestion. What is the most likely diagnosis for this patient's murmur?