A 49-year-old woman presents with increasing shortness of breath on exertion developing over the past three months. She has no chest pain or cough and has noticed no ankle swelling. On examination, blood pressure is 158/61 mmHg, pulse is regular at 88 beats per minute and there are crackles at both lung bases. There is a decrescendo diastolic murmur at the left sternal edge. What is the most likely diagnosis?