A 64-year-old woman presents to the emergency room complaining of shortness of breath, fatigue, palpitations and chest pain. She is not able to lie down on her bed because it makes her shortness of breath worse. On physical examination, you note that she has a holosystolic, high-pitched, blowing murmur with a midsystolic click heard best at the apex with radiation to the axilla. When you used some bedside maneuvers, the murmur decreased in intensity with Valsalva and standing and increased in intensity with handgrip. Which of the following is the most likely diagnosis in this patient?
