Yellow bone marrow can revert to red bone marrow during 

Options

A. Pregnancy
B. Lactation
C. Severe bleeding
D. Childhood

Answer

### Step-by-Step Solution:

1. **Understanding Bone Marrow Types**:
- Bone marrow is a soft, jelly-like substance found in the hollow parts of bones. It plays a crucial role in the production of blood cells.
- There are two types of bone marrow: yellow bone marrow and red (or hematopoietic) bone marrow.

2. **Function of Yellow and Red Bone Marrow**:
- Yellow bone marrow primarily stores fat and can convert to red bone marrow when there is a need for increased blood cell production.
- Red bone marrow is responsible for the production of red blood cells (RBCs), white blood cells, and platelets.

3. **Conditions Leading to Reversion**:
- Yellow bone marrow can revert to red bone marrow under certain conditions that require an increase in blood cell production.
- These conditions include:
- **Severe bleeding**: When there is significant loss of blood, the body needs to produce more RBCs to compensate for the loss.
- Other conditions like pregnancy, lactation, and childhood do not typically cause this reversion.

4. **Conclusion**:
- The primary condition that causes yellow bone marrow to revert to red bone marrow is severe bleeding. This is because the body needs to enhance RBC production to restore blood volume and oxygen-carrying capacity.

### Final Answer:
Yellow bone marrow can revert to red bone marrow during **severe bleeding**.

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