An 11-year-old boy complains of general weakness, fever up to 38,2°C, pain and swelling of the knee joints, feeling of irregular heartbeat. 3 weeks ago, the child had quinsy. Knee joints are swollen, the overlying skin and skin of the knee region is reddened, local temperature is increased, movements are limited. Heart sounds are muffled, extrasystole is present, auscultation reveals apical systolic murmur that is not conducted to the left inguinal region. ESR is 38 mm/h. CRP is 2+, antistreptolysin O titre - 400. What is the most likely diagnosis?