A 42 year old female with no significant past medical history presents to the emergency department with hemoptysis of acute onset. She has noted recent dyspnea on exertion now to the point where she can only walk ½ blocks before having to rest. She denies chest pain. Her temperature is 37.0 C, blood pressure 110/60, heart rate 100, respirations 20, and oxygen saturation 95% on room air. Physical examination reveals normal lung sounds, an irregularly irregular rhythm, a II/IV early diastolic murmur heard best at the cardiac apex, and 1+ lower extremity pitting edema. Laboratory studies are normal. ECG shows sinus tachycardia and left atrial enlargement. Which of the following is causing this patient’s symptoms?