**Question:** All of the following are components of White pulp of spleen, except:

A. Periarteriolar lymphoid sheath (PALS)
B. T cell zone
C. B cell zone
D. Marginal zone

**Correct Answer:** .

**Core Concept:** The white pulp of the spleen is a region responsible for the immune response, primarily involving lymphocytes. It consists of various components that contribute to the immune system's functions, including the processing and presentation of antigens, T cell activation, and B cell differentiation.

**Why the Correct Answer is Right:** Option D, "Marginal zone," is the correct answer because it is not a distinct component of the white pulp. Instead, it is a part of the red pulp surrounding the white pulp, where the interaction between the spleen and circulating blood occurs. The marginal zone is responsible for recognizing and capturing blood-borne pathogens and particles, but it does not contribute to the functions of the white pulp.

**Why Each Wrong Option is Incorrect:**

A. Periarteriolar lymphoid sheath (PALS) is a correct component of the white pulp, containing B cells, dendritic cells, and follicular dendritic cells. PALS is crucial for the processing and presentation of antigens to B cells, leading to their activation and differentiation into plasma cells.

B. T cell zone is another correct component of the white pulp, containing T cells and dendritic cells. This zone is essential for the activation and proliferation of T cells, which play a vital role in cell-mediated immunity.

C. B cell zone is also a correct component of the white pulp, primarily composed of B cells and follicular dendritic cells. This zone is crucial for the activation and differentiation of B cells into plasma cells, producing antibodies in response to antigens.

**Clinical Pearl:** Understanding the spleen's structure and function is essential for clinical reasoning, particularly in cases involving splenomegaly, splenic abscesses, or splenectomy, which can lead to immune deficiencies and increased susceptibility to infections.