A 24-year-old woman presents to your office with a six-month history of progressive exertional dyspnea and intermittent palpitations. She was a competitive swimmer in high school but now finds she gets short of breath walking up two flights of stairs. The palpitations are described as a rapid, regular heartbeat lasting a few minutes and resolving spontaneously. She has not experienced syncope, presyncope, or chest pain. Her past medical history is notable for a heart murmur detected in childhood, which was considered “innocent” and did not prompt further investigation. She takes no medications and has no known allergies. Her family history is noncontributory for cardiac disease.