A 26 year old male is noted to have a V/VI holosystolic murmur associated with a thrill at the left lower sternal border. He has no health complaints and is able to exercise regularly without difficulty. Which of the following is the likely diagnosis?
A. Atrial septal defect
B. Ventricular septal defect
C. Mitral valve regurgitation
D. Tricuspid valve regurgitation