What Rh genotype combination represents a risk for hemolytic disease of the newborn?

The combination of a mother being Rh-negative and a father being Rh-positive poses a risk for hemolytic disease of the newborn due to the potential for Rh incompatibility. When an Rh-negative mother carries an Rh-positive fetus, there is a possibility that the mother's immune system may recognize the Rh-positive blood cells of the fetus as foreign.

If fetal red blood cells enter the mother's circulation, which can occur during pregnancy, labor, or delivery, the mother may produce anti-Rh antibodies. This sensitization typically does not affect the current pregnancy but can have serious consequences in subsequent pregnancies. If the mother has become sensitized and has anti-Rh antibodies, these antibodies can cross the placenta in future pregnancies and attack the red blood cells of an Rh-positive fetus, leading to hemolytic disease of the newborn. This condition can range from mild anemia to severe fetal distress or even stillbirth.

The other combinations listed do not present the same risk. When both parents are Rh-positive or when both are Rh-negative, there is no possibility for Rh incompatibility, as there is no Rh-positive antigen that the mother's immune system could react against.