The most likely diagnosis is pulmonary valve regurgitation. Tetralogy of Fallot comprises a large subaortic ventricular septal defect, infundibular or valvular pulmonary stenosis, aortic override, and right ventricular hypertrophy. Pulmonary regurgitation is the most common structural disorder that occurs following tetralogy of Fallot repair. The clinical findings include features of right heart volume overload with a parasternal (right ventricular) lift and a soft systolic pulmonary outflow murmur. There is a single S₂ because the function of the pulmonary valve is sacrificed during repair. The diastolic murmur heard at the left sternal border that increases in intensity with inspiration is caused by pulmonary valve regurgitation.

Aortic valve regurgitation can occur late in patients following tetralogy of Fallot repair owing to progressive aortic enlargement. The aortic diastolic murmur is generally heard at the left sternal border, but it decreases in intensity with inspiration. A right ventricular prominence would not be expected in a patient with aortic regurgitation.

Recurrent ventricular septal defect also can occur in patients following tetralogy of Fallot repair. However, the physical examination findings would include a systolic murmur heard at the left sternal border, which often obliterates the S₁ and S₂. A right ventricular prominence would not be expected in a patient with a recurrent ventricular septal defect.

Tricuspid valve regurgitation may occur late in patients following tetralogy of Fallot repair owing to progressive right ventricular and annular dilatation from long-standing severe pulmonary valve regurgitation. The physical examination findings include a holosystolic murmur located at the lower left sternal border that increases with inspiration. The systolic murmur in the patient presented here is soft and heard over the pulmonary valve region.