A 3-month-old infant with a late-presenting murmur; which diagnosis is most likely?

• A. Atrial septal defect
• B. Coarctation of the aorta
• C. Patent ductus arteriosus
• D. Ventricular septal defect

Answer: (D)

The key idea here is how the heart defect and the infant’s changing circulation after birth shape when a murmur appears. A ventricular septal defect often has a murmur that depends on the amount of blood crossing the hole. Large VSDs cause symptoms early because of big left-to-right shunts and heart failure, but small defects may be quiet at first. As pulmonary vascular resistance falls over the first weeks to months, more blood can shunt through a small or moderate VSD, revealing a holosystolic murmur best heard along the left lower sternal border. In a 3-month-old, this late appearance of a murmur points toward a VSD rather than a defect that usually presents with a different murmur pattern or earlier signs.

By contrast, a patent ductus arteriosus typically produces a continuous “machinery” murmur that is often evident earlier, especially in term infants. An atrial septal defect tends to show a murmur related to increased flow across the pulmonic valve and can present later in childhood rather than as a new infant murmur. Coarctation of the aorta may have diminished leg pulses or differential blood pressures rather than a characteristic late infant murmur. So the timing and character of the murmur in this scenario best fit a ventricular septal defect.