Review Question - QID 215087

A 78-year-old woman presents to the emergency room after a syncopal ground-level fall. She had been outside doing some strenuous gardening work when she stood up, suddenly felt lightheaded, and lost consciousness. She regained consciousness several seconds later, at which point she was brought to the hospital. Her past medical history is notable for hypertension and osteoporosis for which she takes lisinopril and zoledronate. Past surgical history includes laparoscopic cholecystectomy 3 decades ago. Today, her temperature is 36.5°C (97.7°F), blood pressure is 136/82 mmHg, pulse is 88/min, respirations are 15/min, and oxygen saturation is 96% on room air. On exam, there is a systolic murmur most prominent at the right upper sternal border that radiates to the carotids. Which of the following is the most likely cause of her syncope?