Husband and wife should know their Rh factors because the situation can be serious due to biological incompatibility in one of the following cases :

Options

A. `Rh^(+)` husband and `Rh^(-)` wife
B. `Rh^(-)` husband and `Rh^(-)` wife
C. `Rh^(-)` husband and `Rh^(+)` wife
D. `Rh^(+)` husband and `Rh^(+)` wife

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks why it is important for a husband and wife to know their Rh factors, particularly in the context of biological incompatibility when having a baby.

2. **What is Rh Factor?**: The Rh factor is a protein that can be present on the surface of red blood cells. If a person has this protein, they are Rh positive (Rh+), and if they do not, they are Rh negative (Rh-).

3. **Biological Incompatibility**: Incompatibility occurs when an Rh-negative mother carries an Rh-positive fetus. This can lead to serious complications during pregnancy, especially in subsequent pregnancies.

4. **Erythroblastosis Fetalis**: This is a condition that arises when the mother’s immune system produces antibodies against the Rh-positive blood cells of the fetus. This occurs when the blood of the mother and fetus mix, which can happen during pregnancy or delivery.

5. **Mechanism of Incompatibility**:
- In the first pregnancy, the chances of blood mixing are lower, but if the fetus is Rh-positive and the mother is Rh-negative, the mother’s body may start to produce antibodies against the Rh factor.
- In subsequent pregnancies, if the fetus is again Rh-positive, the mother’s antibodies can cross the placenta and attack the fetus’s red blood cells, leading to hemolysis (destruction of red blood cells).

6. **Identifying the Risky Situation**:
- If the husband is Rh positive and the wife is Rh negative, there is a high chance that the fetus will be Rh positive. This situation poses a risk of erythroblastosis fetalis.
- If both parents are Rh negative or if the husband is Rh negative and the wife is Rh positive, there is no risk of incompatibility.

7. **Conclusion**: Therefore, the serious situation due to biological incompatibility occurs when the husband is Rh positive and the wife is Rh negative.

### Final Answer:
The situation that can lead to serious biological incompatibility is when the husband is Rh positive and the wife is Rh negative.

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