Valvular Heart Disease USMLE Step 1 Practice Question

A 42-year-old woman presents to the clinic with a 3-year history of exertional dyspnea and occasional palpitations. She denies syncope or chest pain. Her mother died suddenly at age 58 from an unknown cardiac cause. On physical examination, blood pressure is 118/76 mmHg, heart rate is 78 bpm and regular. Cardiac auscultation reveals a midsystolic click at the apex followed by a late systolic murmur. When the patient stands from a squatting position, the click occurs earlier and the murmur lengthens. Transthoracic echocardiography shows billowing of both mitral valve leaflets superior to the mitral annular plane during ventricular systole, with mild mitral regurgitation. Left ventricular systolic function is normal. Which of the following is the most likely diagnosis?