A 75-year-old man is bought to the hospital because of a syncopal episode. There was no incontinence or post-event confusion. On examination, his blood pressure is 140/80 mm Hg, pulse 72/min with no postural changes. His second heart sound is diminished and there is a systolic ejection murmur that radiates to the carotids. With the Valsalva maneuver, the murmur decreases in length and intensity.For the above patient with a systolic murmur, select the most likely diagnosis.