A 25-year-old male presents with palpitations and shortness of breath for 3 years and fever for 1 month. Examination: temperature 37.6°C, petechiae on conjunctiva, clubbing, heart rate 100 bpm, biphasic murmur at the apex, diastolic murmur at the aortic valve area, splenomegaly 1 cm below the costal margin, hemoglobin 80 g/L, urinary protein (+). The most likely diagnosis is: