A 58-year-old woman with a history of a previous myocardial infarction and mild mitral regurgitation presents to her cardiologist for a routine office visit. Her only medicines are aspirin and hydrochlorothiazide for mild hypertension. She is asymptomatic, and her physical examination is remarkable for a grade 3/6 late systolic murmur heard best at the apex. The result of the physical examination is normal except for an irregular pulse. An ECG was obtained.What is the diagnosis?