In 2 weeks after pharyngitis a 25-year-old patient started complaining of rise in temperature to 38°С, fatigue, shortness of breath during walking, migrating joint intumescence and pain. On physical exam: cyanosis of lips, pulse is weak, rhythmical, 100 bpm. The left heart border is moved outwards from mediaclavicular line for 1 cm. Sound I on the apex is weakened, soft systolic murmur is auscultated. What etiologic factor caused this pathological process in the most probable way?