What is the primary cause of erythroblastosis fetalis in a second pregnancy?

Erythroblastosis fetalis, also known as hemolytic disease of the newborn, primarily occurs due to Rh incompatibility in a second pregnancy. This condition arises when an Rh-negative mother is pregnant with an Rh-positive baby, inherited from an Rh-positive father. In the first pregnancy, if the mother is sensitized to the Rh factor (usually during delivery or due to a prior miscarriage or abortion), her immune system produces antibodies against Rh-positive blood cells.

If the mother becomes pregnant again with another Rh-positive baby, these pre-formed antibodies can cross the placenta and attack the fetal red blood cells, leading to hemolysis. This results in anemia and can cause significant complications for the fetus, including jaundice and even heart failure.

While other options such as maternal diabetes, infections, and chromosomal abnormalities can affect pregnancy and fetal development, they do not specifically relate to the mechanism of erythroblastosis fetalis following Rh incompatibility. Thus, the predominant cause in a second pregnancy, particularly after a sensitization event, is indeed hemolytic disease due to Rh incompatibility.