**Core Concept**
Hemolytic disease of the newborn (HDN) is a condition where the baby's red blood cells are destroyed faster than they can be made, often due to maternal antibodies against the baby's RhD antigen or other blood group antigens. This condition is a result of the incompatibility between the mother's and the baby's blood types, leading to hemolysis and potential jaundice in the newborn.

**Why the Correct Answer is Right**
In HDN, the maternal antibodies against the RhD antigen (or other blood group antigens) cross the placenta and bind to the baby's red blood cells, marking them for destruction by the baby's immune system. This process is facilitated by the presence of complement system, which is a group of proteins that enhance the ability of antibodies to destroy cells. The destruction of red blood cells results in the release of bilirubin, a pigment that can accumulate in the baby's brain, leading to kernicterus if left untreated.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because HDN is not caused by maternal antibodies against the baby's ABO blood group antigens, although ABO incompatibility can cause hemolytic disease of the newborn. However, it is less severe than RhD incompatibility.

**Option B:** This option is incorrect because HDN can be caused by maternal antibodies against the baby's RhD antigen, but it is not the only cause. Other blood group antigens such as Kell and Duffy can also cause HDN.

**Option C:** This option is incorrect because HDN can occur in babies who are not exposed to maternal antibodies through the placenta. This can happen if the mother is not sensitized to the baby's blood group antigens during a previous pregnancy.

**Option D:** This option is incorrect because HDN is not caused by maternal antibodies against the baby's HLA antigens. HLA (human leukocyte antigen) incompatibility can cause graft-versus-host disease, but it is not associated with HDN.

**Clinical Pearl / High-Yield Fact**
The RhD antigen is the most common cause of HDN, and sensitization to this antigen can occur during pregnancy, childbirth, or blood transfusions. Pregnant women who are RhD negative should receive RhD immune globulin to prevent sensitization.

**Correct Answer: A.**