Which of the following is primary lymphoid organ ?

Options

A. Liver
B. Spleen
C. Bone marrow
D. Yolk sac

Answer

To determine which of the given options is a primary lymphoid organ, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Definition of Primary Lymphoid Organs**:
- Primary lymphoid organs are those where lymphocytes (B and T cells) are formed and mature. These organs provide an environment for stem cells to divide and differentiate into lymphocytes.

2. **Identify the Options**:
- The options given are:
- A) Liver
- B) Spleen
- C) Bone Marrow
- D) Yolk Sac

3. **Evaluate Each Option**:
- **Liver**: The liver is primarily involved in metabolism and detoxification, not in the formation or maturation of lymphocytes. Therefore, it is not a primary lymphoid organ.
- **Spleen**: The spleen is involved in filtering blood and is considered a secondary lymphoid organ, where immune responses are initiated. It does not serve as a site for lymphocyte maturation.
- **Bone Marrow**: Bone marrow is a primary lymphoid organ where hematopoiesis occurs, and it is the site for the formation and maturation of B cells. It is one of the two primary lymphoid organs.
- **Yolk Sac**: The yolk sac is an embryonic structure that contributes to early blood cell formation but is not classified as a primary lymphoid organ.

4. **Conclusion**:
- Based on the evaluation, the only option that qualifies as a primary lymphoid organ is **C) Bone Marrow**.

### Final Answer:
The primary lymphoid organ among the options provided is **Bone Marrow**.

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