Heart Failure USMLE Step 1 Practice Question

A 27-year-old woman presents with intermittent palpitations and atypical chest discomfort. She denies dyspnea on exertion. Vital signs: BP 118/76 mmHg, HR 92/min, RR 16/min, temp 37°C. Cardiac auscultation reveals a midsystolic click followed by a late systolic murmur at the apex. Echocardiography shows superior displacement of the mitral leaflet into the left atrium during systole. She takes no cardiac medications and has no history of hypertension or coronary disease. Which diagnosis best explains these findings?