A 73-year-old woman had an episode a week ago in which she became disoriented, had difficulty speaking, and had persisting weakness on the right side of her body On phys- ical examination, she is now afebrile with pulse of 68/min, res pirations of 15/min, and blood pressure of 130/85 mm Hg. On auscultation, the lungs are clear, the heart rate is irregular, and there is a midsystolic click. a chest CT scan shows a focus of bright attenuation within the heart. An echocardiogram shows that one valvular leaflet appears to balloon upward. The ejec- tion fraction is estimated to be 55%. Laboratory findings show serum creatine kinase (CK), 100 U/L; glucose, 77 mg/dL; creatinine, 0.8 mg/dL; calcium, 8.1 mg/dL; and phosphorus, 3.5 mg/dL. Which of the following is the most likely diagnosis?