A 38-year-old man from rural India presents with fever for 3 weeks, progressive dyspnea, and a new diastolic murmur at the left sternal border. He has a history of rheumatic heart disease diagnosed 10 years ago. On examination: temperature 38.5°C, heart rate 102/min, blood pressure 140/80 mmHg, splinter hemorrhages on fingernails, and Osler nodes on fingertips. Blood cultures (before antibiotics) grow alpha-hemolytic streptococci. Transthoracic echocardiography shows a 12 mm vegetation on the aortic valve with mild aortic regurgitation. What is the most likely diagnosis according to the modified Duke criteria?