What condition is primarily associated with late cyanosis?

Late cyanosis in the context of congenital heart disease is primarily associated with conditions that result in unoxygenated blood entering the systemic circulation after the newborn period. In this case, a ventricular septal defect (VSD) can lead to late cyanosis if significant pulmonary hypertension develops over time.

Initially, a small to moderate VSD may not cause much mixing of blood and may not lead to cyanosis because the left-to-right shunt allows oxygenated blood from the left ventricle to flow into the right ventricle and then into the pulmonary arteries. However, as the child grows, increased blood flow to the lungs can lead to pulmonary vascular resistance. If the pulmonary vascular resistance becomes higher than the systemic resistance, right-to-left shunting can occur through the VSD, causing deoxygenated blood from the right ventricle to mix with the systemic circulation, resulting in late cyanosis.

This understanding of blood flow dynamics explains why a ventricular septal defect is associated with late cyanosis in certain cases, particularly when there is development of pulmonary hypertension and a resultant shunt reversal. Other conditions like cyanotic heart diseases generally present with early cyanosis because they allow deoxygenated blood to enter the systemic circulation from birth.