Which type of congenital heart defect is associated with right-to-left shunt?

A right-to-left shunt in congenital heart defects means that blood flows from the right side of the heart to the left side, bypassing the lungs, which can lead to systemic cyanosis due to deoxygenated blood entering the arterial circulation.

Transposition of the great arteries is a condition in which the aorta and pulmonary artery are switched. This abnormal connection results in two separate circulations: one systemic (where deoxygenated blood returns to the body) and one pulmonary (where oxygenated blood returns to the lungs). Because of this arrangement in the absence of a shunt, the systemic circulation does not receive oxygenated blood, leading to cyanosis. A right-to-left shunt can occur when there are additional connections, such as an atrial septal defect or ventricular septal defect, which allow mixing of the blood to some extent. However, the primary defect itself, transposition of the great arteries, is characterized by this significant right-to-left shunting under certain conditions, especially if there is a associated defect.

In contrast, an atrial septal defect and a ventricular septal defect typically lead to left-to-right shunting because they allow oxygen-rich blood from the left heart to flow back into the right heart, leading to