During a routine exam, what heart sound characteristic is noted in a five-year-old with a likely atrial septal defect (ASD)?

In a child with an atrial septal defect (ASD), the characteristic heart sound that is most notable is a fixed and widely split S2. This occurs due to the increased blood flow from the left atrium to the right atrium, which subsequently increases the volume of blood that flows into the right ventricle and pulmonary artery during systole.

As this extra volume enters the right side of the heart, it causes a delay in the closure of the pulmonic valve, leading to a split S2. The term "fixed" implies that this splitting does not vary with respiration, which is typically the case in children with ASD. The fixed nature of the split S2 is a direct result of the continuous left-to-right shunt that characterizes the defect, leading to increased right heart volume overload.

This understanding highlights why option B is the correct choice, as this heart sound finding is both specific and common in children with an ASD.