## **Core Concept**
CD99 is a cell surface glycoprotein that is often used as a diagnostic marker in pathology. It is particularly associated with Ewing's sarcoma, a type of malignant bone tumor. The CD99 antigen is encoded by the MIC2 gene and is widely used in immunohistochemistry to help diagnose Ewing's sarcoma.

## **Why the Correct Answer is Right**
The correct answer, Ewing's sarcoma, is associated with CD99 because this tumor characteristically expresses high levels of the CD99 protein on its cell surface. The detection of CD99 by immunohistochemistry is a key diagnostic tool for Ewing's sarcoma, helping to differentiate it from other types of sarcomas and tumors. The specificity of CD99 for Ewing's sarcoma makes it a valuable marker in pathology.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, although CD99 can be expressed in some other types of tumors and normal tissues, its specific association with Ewing's sarcoma makes it a standout marker for this particular condition.
- **Option B:** This option is incorrect as there is no commonly recognized association of CD99 with this specific condition in the context provided.
- **Option C:** This option is incorrect for similar reasons as option B; there's no well-established link with CD99 that is relevant to the question.
- **Option D:** This option is incorrect as it does not accurately represent the primary association of CD99.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that while CD99 is highly expressed in Ewing's sarcoma, it is not exclusively specific to this tumor and can be found in some other conditions, such as some lymphomas and in normal tissues. However, in the context of a small blue round cell tumor, CD99 positivity is highly suggestive of Ewing's sarcoma.

## **Correct Answer:** . Ewing's Sarcoma