What is the most probable explanation for an infant with a weakly positive direct antiglobulin test and anti-D present in the mother's serum?

The situation described involves an infant with a weakly positive direct antiglobulin test (DAT) and the presence of anti-D in the mother's serum. This presentation is characteristic of Rh hemolytic disease of the fetus and newborn (HDFN), where maternal antibodies to the D antigen can cross the placenta and cause hemolysis in the fetus.

In this scenario, option C suggests that the infant has a false negative Rh typing, which can occur if there is a small amount of fetal Rh-positive blood present in the infant's circulation. The weakly positive DAT indicates that there is some degree of sensitization occurring, but not strongly enough to suggest severe Rh disease. A false negative Rh typing could result if the fetal cells are overwhelmed by maternal cells or if there is a low level of fetal Rh-positive blood that does not trigger a strong response in testing.

The presence of maternal anti-D indicates that the mother has been sensitized, likely due to exposure to Rh-positive blood from a previous pregnancy or transfusion. If the infant's blood type is misclassified due to this low-level presence of maternal anti-D, it can mask the actual Rh typing results. Therefore, this option appropriately aligns with the mother's sensitization and the infant's clinical presentation.