During a routine athletic physical, a 15-year-old boy is fou…

During a routine athletic physical, a 15-year-old boy is found to have a systolic thrill that is palpable at the lower left sternal border accompanied by a harsh, pansystolic murmur that is heard best at the site of the thrill. He is asymptomatic and has no evidence of hypertension, cyanosis, or edema. An electrocardiogram and a chest radiograph are normal. What is the most likely diagnosis?