Review Question - QID 109690

An 84-year-old man presents to the emergency department for a loss of consciousness. He was using the bathroom when he lost consciousness and fell, hitting his head on the counter. The patient has a medical history of diabetes, hypertension, obesity, factor 4 Leiden, constipation, myocardial infarction, and vascular claudication. His current medications include lisinopril, atorvastatin, valproic acid, propranolol, insulin, metformin, and sodium docusate. The patient denies the use of illicit substances. His temperature is 99.5°F (37.5°C), blood pressure is 167/98 mmHg, pulse is 90/min, respirations are 15/min, and oxygen saturation is 98% on room air. Physical exam reveals an elderly man sitting comfortably in his stretcher. Cardiac exam reveals a systolic murmur heard at the right upper sternal border that radiates to the carotids. Pulmonary exam reveals mild bibasilar crackles. Neurological exam reveals 5/5 strength in his upper and lower extremities with normal sensation. The patient's gait is mildly unstable. The patient is unable to give a urine sample in the emergency department and states that he almost fainted again when he tried to. Which of the following is the most likely diagnosis?