A 6-month-old infant is diagnosed with a congenital heart defect that causes a left-to-right shunt, resulting in increased pulmonary blood flow. Which of the following defects is most likely?

A. Coarctation of the Aorta is incorrect because this defect causes obstruction of blood flow from the left ventricle to the aorta. It is a pressure load problem (left ventricular hypertension) rather than a left-to-right shunt, and it does not primarily increase pulmonary blood flow. B. Tetralogy of Fallot is incorrect because this is a cyanotic defect characterized by right-to-left shunting due to pulmonary stenosis and a VSD. Pulmonary blood flow is often decreased rather than increased. C. Transposition of the Great Arteries is incorrect because this is a cyanotic defect in which the aorta arises from the right ventricle and the pulmonary artery from the left ventricle, resulting in parallel circulation. Pulmonary blood flow is not increased by a left-to-right shunt. D. Ventricular Septal Defect (VSD) is correct because it is a acyanotic defect that allows blood to flow from the left ventricle (higher pressure) to the right ventricle (lower pressure). This left-to-right shunt increases pulmonary blood flow, which can lead to symptoms such as tachypnea, poor weight gain, and frequent respiratory infections. VSDs are the most common congenital heart defect in infants.