Organ System Pathology USMLE Step 1 Practice Question

A 27-year-old woman presents with intermittent palpitations and atypical chest pain. Vital signs: BP 118/76 mmHg, HR 88 bpm, RR 16/min, temp 37°C, SpO2 98% on room air. Cardiac examination reveals a midsystolic click followed by a late systolic murmur that intensifies when standing from a squat position. Chest X-ray shows normal cardiac silhouette. She denies dyspnea. Echocardiography demonstrates superior leaflet prolapse with mild mitral regurgitation. Which diagnosis best explains these findings?