Which is not a secondary lymphoid organ?

Options

A. Thymus
B. Spleen
C. Appendix
D. MALT

Answer

To determine which option is not a secondary lymphoid organ, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Lymphoid Organs**:
- Lymphoid organs are classified into two categories: primary and secondary.
- Primary lymphoid organs are where lymphocytes originate and mature. The primary organs include the thymus and bone marrow.

2. **Identify Secondary Lymphoid Organs**:
- Secondary lymphoid organs are where lymphocytes are activated and proliferate. Examples include the spleen, lymph nodes, tonsils, and the appendix.

3. **Examine the Options**:
- **Option A: Thymus**: This is a primary lymphoid organ, not secondary.
- **Option B: Spleen**: This is a secondary lymphoid organ.
- **Option C: Appendix**: This is also considered a secondary lymphoid organ.
- **Option D: MALT (Mucosal Associated Lymphoid Tissue)**: This is not classified as an organ but rather as a tissue.

4. **Conclusion**:
- Since the thymus is a primary lymphoid organ and MALT is not classified as a secondary lymphoid organ, the correct answer to the question "Which is not a secondary lymphoid organ?" is **Option D: MALT**.

### Final Answer:
**Option D: MALT (Mucosal Associated Lymphoid Tissue)** is not a secondary lymphoid organ.

---