An 86-year-old man has had increasing dyspnea and reduced exercise tolerance for the past 7 years. On physicalexamination, he is afebrile and has a blood pressure of 135/85 mm Hg. An irregularly irregular heart rate averaging 76/min is audible on auscultation of the chest. Crackles are heard at the bases of the lungs. A chest radiograph shows mild cardiomegaly and mild pulmonary edema. Echocardiography shows slight right and left ventricular wall thickening with reduced left and right ventricular wall motion, reduced left ventricular filling, and an ejection fraction estimated to be 25% An endomyocardial biopsy specimen shows amorphous pink-staining deposits between myocardial fibers, but no inflammation and no necrosis. Which of the following is the most likely diagnosis? (A) Cardiac amyloidosis (B) Rheumatic heart disease (C) Constrictive pericarditis (D) Mitral valve prolapse (E) Left ventricular aneurysm