An 11-year-old boy had a sore throat, no cough, tonsillar exudates, and 38.3deg C 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 diastolic mitral murmur is audible, and there are diffuse rales over both lungs. Over the next 2 days, he 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 is most likely to be the cause of the left ventricular failure?