A 16-year-old boy is brought to the physician because of a 5-day history of fever, rash on his stomach, and pain of his wrists and knees. Two weeks ago, he was diagnosed with streptococcal pharyngitis, and he was treated with a 10-day course of penicillin. His temperature is 38.8°C (101.8°F), pulse is 92/min, and respirations are 23/min. Physical examination shows a nonpruritic, mildly raised pink abdominal rash with distinct borders. A mitral regurgitation murmur is heard. Palpation of the wrists and knees produces pain. Which of the following pathologic changes is most likely in this patient's mitral valve at this time? A) Calcification and fibrosis within the valve cusps, B) Fibrinoid necrosis along the lines of closure, C) Fibrosis and calcification of the chordae tendineae, D) Fibrosis of the valve body, E) Myxomatous degeneration of the valve body