## **Core Concept**
Ewing's Sarcoma is a type of malignant bone tumor that primarily affects children and young adults. It is characterized by the presence of specific genetic translocations, most commonly the EWSR1-FLI1 fusion. Immunohistochemistry plays a crucial role in the diagnosis of Ewing's Sarcoma by helping to identify specific markers.

## **Why the Correct Answer is Right**
The correct answer, **CD99 (MIC2)**, is a cell surface glycoprotein that is widely used as a diagnostic marker for Ewing's Sarcoma. The MIC2 gene product, CD99, is expressed in about 90% of Ewing's Sarcoma cases, making it a sensitive marker for this tumor. The positivity of CD99, in conjunction with clinical and radiological findings, supports the diagnosis of Ewing's Sarcoma. The EWSR1-FLI1 fusion protein, resulting from the t(11;22) translocation, is also a specific genetic marker but is detected through molecular techniques like PCR or FISH, not immunohistochemistry.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain neuroendocrine markers might be positive in some cases of Ewing's Sarcoma, they are not specific or sensitive enough to be used as primary diagnostic markers.
- **Option B:** This option might represent a marker for another type of tumor and is not specifically associated with Ewing's Sarcoma.
- **Option D:** Similarly, this could be a marker for a different malignancy and is not characteristically positive in Ewing's Sarcoma.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that while CD99 is highly sensitive for Ewing's Sarcoma, it is not entirely specific. Other tumors, such as some lymphomas and synovial sarcomas, can also express CD99. Therefore, the diagnosis of Ewing's Sarcoma relies on a combination of immunohistochemical findings, molecular studies (for the EWSR1 rearrangement), and clinical/radiological correlation.

## **Correct Answer:** . CD99 (MIC2)