Q. A 41-year-old female presents to the emergency department with complaints of chest pain, shortness of breath, and general malaise. She reports not feeling well for the past few weeks. The patient states that approximately 2 hours ago she suddenly developed chest pain and felt as though her heart was “racing”. She also describes a recent history of weight loss, decreased appetite, and difficulty lying flat without becoming short of breath. Additionally, the patient reports two episodes within the past month of headaches with associated unilateral facial paresthesia, both of which resolved without intervention. She is under more stress than usual at work, and she believes her symptoms are “probably just anxiety”. Her past medical history is significant for two uncomplicated deliveries of term infants, and is otherwise unremarkable. She denies tobacco use and drinks alcohol occasionally. On physical examination, her temperature is 38.3 degrees Celsius (101 degrees Fahrenheit), blood pressure is 140/90 mm Hg, heart rate 190/min, respiratory rate 25/min, and her oxygen saturation is 97 percent on room air. Her pupils are equal, round, and reactive to light and accommodation. No focal neurologic deficits are detected. Her neck is supple without jugular venous distention. Cardiac assessment reveals tachycardia and a high-pitched, holosystolic sound best heard at the apex of the heart with the patient in the left lateral decubitus position. The sound can also be heard at the left sternal border. Additionally, a low-pitched, early diastolic “plopping” sound is heard immediately after the second heart sound. No rubs or gallops are detected. No masses are detected in her abdomen, and her pulses are equal in all extremities. Her remaining physical examination is unremarkable. You order an electrocardiogram (see image). Based on the information you have at this point in time, what is the most likely underlying cause of this patient’s symptoms?