An 18-year-old female presents to the medicine OPD with a history of fever and joint pain, and a sore throat 3 weeks prior. On examination, she is febrile. Laboratory tests show CRP of 54 mg/L, ESR of 90 mm/hr, and ASO titer of 750 IU/ml. Transoesophageal echocardiography reveals severe mitral stenosis, regurgitation, severe aortic regurgitation, thickening and restricted motion of the posterior mitral valve leaflet, and dilation of the left atrium. Discuss the likely diagnosis and explain the immunopathogenesis of this condition.