In developing foetus ,erthoblastosis foetalis is caused by :

Options

A. haemolysis
B. clumping of RBCs
C. phagocytosis by WBCs
D. failure of blood clotting

Answer

### Step-by-Step Solution:

1. **Understanding Erythroblastosis Fetalis**: Erythroblastosis fetalis is a condition that occurs in a developing fetus when there is an incompatibility between the blood types of the mother and the fetus. This is most commonly associated with Rh incompatibility.

2. **Rh Incompatibility**: This condition arises when the mother is Rh-negative (lacking the Rh factor) and the father is Rh-positive (having the Rh factor). If the fetus inherits the Rh-positive blood type from the father, the mother’s immune system may recognize the Rh-positive cells as foreign.

3. **Immune Response**: The mother’s body produces antibodies against the Rh-positive antigen present in the fetus's red blood cells (RBCs). This immune response is particularly significant if the mother has been sensitized by a previous pregnancy with an Rh-positive child.

4. **Consequences for the Fetus**: The antibodies produced by the mother can cross the placenta and attack the fetal RBCs, leading to their destruction. This process is known as hemolysis.

5. **Identifying the Correct Option**:
- Option A: Hemolysis in erythroblastosis fetalis - This is correct as it describes the main pathological process.
- Option B: Clumping of RBCs - This is incorrect; clumping is not the primary issue in erythroblastosis fetalis.
- Option C: Phagocytosis by WBCs - This is also incorrect; while WBCs do play a role in immune responses, the primary issue is hemolysis, not phagocytosis.
- Option D: Failure of blood clotting - This is incorrect; this condition is related to hemophilia, not erythroblastosis fetalis.

6. **Conclusion**: The correct answer is option A: hemolysis.