**Core Concept**
The question is testing the understanding of the pathophysiology of hemolytic disease of the newborn (HDN), specifically the role of blood group incompatibility between the mother and the fetus. In HDN, maternal antibodies against fetal red blood cells can cause hemolysis, leading to jaundice.

**Why the Correct Answer is Right**
In this scenario, the mother is blood group O, which is naturally devoid of A and B antigens. However, she has developed antibodies against A antigens due to previous exposure, a phenomenon known as isoimmunization. These antibodies can cross the placenta and bind to the A antigens on the surface of the baby's red blood cells, leading to hemolysis. The baby's blood group A status confirms the presence of A antigens on his red blood cells, making this a classic case of ABO incompatibility.

**Why Each Wrong Option is Incorrect**
* **Option A:** Rh incompatibility is a different cause of HDN, typically involving the mother being Rh-negative and developing antibodies against the Rh-positive fetus. This is not relevant in this scenario.
* **Option B:** G6PD deficiency is a genetic disorder that affects red blood cell metabolism, leading to hemolytic anemia, but it is not related to blood group incompatibility.
* **Option D:** Hemolytic disease of the newborn can also be caused by other blood group antigens, such as Kell or Duffy, but the scenario described is most consistent with ABO incompatibility.

**Clinical Pearl / High-Yield Fact**
Remember that ABO incompatibility is the most common cause of hemolytic disease of the newborn, and it typically presents with jaundice in the first few days of life.

**Correct Answer:** B. ABO incompatibility due to maternal isoimmunization.