An 8-year-old boy is brought to the pediatric clinic by his mother because of a 3-day history of fever and joint pain. The mother reports that the pain started in his right knee and completely resolved after 2 days, then his left ankle became painful and swollen, and now only his wrists are affected. He has been complaining that the joints feel hot. Three weeks ago, he had a severe sore throat with difficulty swallowing that lasted about 5 days but resolved without treatment. The family could not afford to take him to the doctor at that time. He has no other medical history and takes no medications. His temperature is 38.4°C (101.1°F), pulse is 110/min, respirations are 20/min, and blood pressure is 95/60 mm Hg. Physical examination shows a well-appearing boy in mild distress. His wrists are warm and swollen with tenderness to palpation. Cardiac examination shows a regular rhythm and a grade 2/6 pansystolic murmur heard best at the apex that was not present at his well-child visit 6 months ago. A friction rub is heard along the left sternal border. The lungs are clear to auscultation bilaterally. Laboratory studies show a leukocyte count of 12,500/mm³, erythrocyte sedimentation rate of 45 mm/hr, and antistreptolysin O titer of 420 IU/mL (normal <200 IU/mL). Blood cultures obtained show no growth at 48 hours. Urinalysis shows no blood, no protein, and no casts.
Which of the following is the most likely diagnosis?