A 12-year-old boy was brought to the physician with a sore throat and fever 3 weeks ago, and a throat culture was positive for group A B-hemolytic streptococcus. On the follow-up examination, the child is afebrile. His pulse is 85/min, respirations are 18/min, and blood pressure is 90/50 mm Hg. On auscultation, a murmur of mitral regurgitation is audible, and there are diffuse rales over both lungs. The child is admitted to the hospital and over the next 2 days has several episodes of atrial fibrillation accompanied by signs of acute left ventricular failure. Which of the following pathologic changes occurring in this child's heart during hospitalization is most likely to be the cause of the left ventricular failure? (A) Amyloidosis (B) Endocardial fibroelastosis (B) Endocardial fibroelastosis (C) Fibrinous pericarditis (D) Fibrosis of mitral valve with fusion of commissures (E) Myocarditis (F) Tamponade (G) Verrucous endocarditis