## **Core Concept**
The question revolves around the concept of **ABO blood group inheritance** and **heteropaternal superfecundation** or **heteropaternal superfetation**. ABO blood groups are determined by a single gene with three types of alleles: A, B, and O. The A and B alleles are codominant, while the O allele is recessive.

## **Why the Correct Answer is Right**
The correct answer, **D. Heteropaternal superfecundation**, explains the scenario where a woman, whose blood type is O, and her husband, also type O, have twins with discordant blood types (one type O and one type B). Blood type O is recessive and can be represented as OO. For the couple to have a child with blood type B, the B allele must come from somewhere. Since the husband is type O (OO), he can only contribute an O allele. Therefore, the B allele must come from another male, making **heteropaternal superfecundation** (the fertilization of two ova from the same woman by sperm from different men) the only plausible explanation. This phenomenon allows for the possibility of twins having different fathers.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because **germline mosaicism** refers to a situation where a portion of the gametes (sperm or eggs) carry a mutation not present in the rest of the individual's cells or other gametes. This does not directly explain the inheritance of a B allele in a scenario where both parents are type O.
- **Option B:** **Maternal cell contamination** refers to the presence of maternal cells in a sample taken from a fetus or newborn, which can lead to incorrect genotyping. This does not explain the presence of a B allele in one of the twins.
- **Option C:** **Chimerism** refers to the presence of two or more different cell populations within an individual, which could result from the fusion of two fertilized eggs. While this could result in mixed blood types, it doesn't specifically explain how a B allele would appear in a family where both parents are type O without invoking another male as the source of the B allele.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **heteropaternal superfecundation** can lead to twins with different biological fathers, which might have implications for paternity testing and family dynamics. This phenomenon, although rare, underscores the complexity of human reproduction.

## **Correct Answer: D. Heteropaternal superfecundation**