**Core Concept:** Thymoma and lymphoma are two types of tumors that can affect the mediastinum, but they have distinct histological and immunohistochemical features. Thymoma is a tumor originating from the epithelial cells of the thymus, while lymphoma is a tumor arising from lymphoid cells. Understanding these features is crucial for accurate diagnosis and treatment planning.

**Why the Correct Answer is Right:** Thymoma is typically characterized by the presence of thymic epithelial cells, specifically type B3 cells, which stain positively for CD5 and TdT (T cell development markers). In contrast, lymphomas primarily consist of lymphoid cells and often express markers like CD20, CD79a, and Bcl-2, which are absent in thymoma.

**Why Each Wrong Option is Incorrect:**

A. Thymoma can express CD20 and CD79a markers in rare cases; however, it is not the primary marker for distinction between thymoma and lymphoma.

B. TdT is also expressed in some lymphomas, particularly in Burkitt's lymphoma, making it an inadequate marker for distinguishing between thymoma and lymphoma.

C. CD5 is a marker expressed in T cell lymphomas, but it can also be expressed in thymoma, making it a less specific marker for distinguishing between the two entities.

D. CD2 is a marker expressed in some T cell lymphomas but is not specific to either thymoma or lymphoma, making it an unsuitable marker for distinction.

**Clinical Pearl:** A panel of markers, including CD5, TdT, CD20, CD79a, and CD3, is often employed to differentiate thymoma from lymphoma. This comprehensive approach increases the specificity and accuracy of distinguishing these two entities.

**Correct Answer:** D. CD2 is a marker expressed in some T cell lymphomas but is not specific to either thymoma or lymphoma, making it an unsuitable marker for distinction.