Valvular Heart Disease USMLE Step 1 Practice Question

A 72-year-old man with a 40-year history of hypertension presents to clinic with progressive dyspnea on exertion that began 3 months ago. He reports he can no longer walk up one flight of stairs without stopping to catch his breath. He denies orthopnea or paroxysmal nocturnal dyspnea. On physical examination, blood pressure is 165/92 mmHg, heart rate 78 bpm, and respiratory rate 16/min. Cardiac auscultation reveals a single loud S2, a prominent S4 gallop, and a harsh crescendo-decrescendo systolic ejection murmur best heard at the right upper sternal border with radiation to the bilateral carotids. Carotid upstrokes are delayed and diminished bilaterally. Echocardiography demonstrates severe left ventricular hypertrophy with a small left ventricular cavity, a peak aortic valve gradient of 65 mmHg, and preserved left ventricular ejection fraction. Which of the following is the most likely diagnosis?