A 60 year old female with no significant past medical history presents with increasing dyspnea on exertion and chest pains with exertion. All of her symptoms are relieved with rest. Her temperature is 37.0, blood pressure 120/80, heart rate 80, respirations 20, and oxygen saturation 95% on room air. Physical examination reveals a late-peaking II/VI systolic ejection murmur at the right upper sternal border radiating to her carotid arteries, a soft S2 heart sound, and an S4 is present. Laboratory studies are normal. ECG reveals normal sinus rhythm with left ventricular hypertrophy. Which of the following is likely the culprit for this patient’s symptoms?