**Core Concept**
MIC-2, also known as CD99, is a cell surface glycoprotein that serves as a marker for certain types of cancer. It is a member of the leukosialin/CD62L cluster and plays a role in cell-to-cell adhesion and signaling.

**Why the Correct Answer is Right**
MIC-2 is specifically expressed in Ewing sarcoma, a malignant bone tumor that primarily affects children and young adults. The MIC-2 gene, which encodes the CD99 protein, is involved in the pathogenesis of Ewing sarcoma. The expression of MIC-2 has been found in approximately 90% of Ewing sarcoma cases, making it a valuable diagnostic marker for this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** While Ewing sarcoma is the correct answer, it's essential to understand that MIC-2 is not a marker for other types of sarcomas, such as osteosarcoma or rhabdomyosarcoma. These tumors may express different markers, such as S100 or desmin.
**Option B:** Chronic lymphocytic leukemia (CLL) is a type of cancer that affects the blood and bone marrow, but it is not associated with MIC-2 expression. CLL is typically characterized by the expression of other markers, such as CD5 and CD23.
**Option C:** Mantle cell lymphoma is a type of non-Hodgkin lymphoma that affects the lymph nodes and spleen. While it does express certain markers, such as CD20 and CD5, MIC-2 is not one of them.

**Clinical Pearl / High-Yield Fact**
When diagnosing Ewing sarcoma, it's essential to remember that MIC-2 expression is a key feature of this tumor type. A positive reaction for MIC-2 in a bone tumor can help differentiate Ewing sarcoma from other types of sarcomas and lymphomas.

**✓ Correct Answer: A. Ewing sarcoma**