Review Question - QID 210414

A 67-year-old man presents to his primary care physician for a wellness checkup. He has been doing well and currently has no concerns. The patient's daughter notes that she feels he is abnormally fatigued and has complained of light-headedness whenever he gardens. He also admits that he fainted once. The patient has a medical history of type 2 diabetes, hypertension, and constipation. He recently had a "throat cold" that he recovered from with rest and fluids. His temperature is 98.9°F (37.2°C), blood pressure is 167/98 mmHg, pulse is 90/min, respirations are 12/min, and oxygen saturation is 99% on room air. Physical exam reveals a systolic murmur heard best along the right upper sternal border. An ECG is performed and demonstrates no signs of ST elevation. Cardiac troponins are negative. Which of the following is the most likely cause of this patient's symptoms?