A 12-year-old girl complains of general weakness, rise of body temperature up to 38,2°C, pain and swelling of knee joints, feeling of cardiac rhythm disruption. The child had tonsillitis 3 weeks ago. The knee joints are swollen, local raise of temperature is observed, mobility is reduced. Heart sounds are weakened, extrasystole is present; at the cardiac apex systolic noise can be auscultated, which is not conducted to the left axillary region. ESR is 38mm/h. CRP2+. antistreptolysin-O titers are 400. The most likely disease is: