Which of the following is a specific marker for ewings sarcoma?
## **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 and molecular markers. The diagnosis of Ewing's sarcoma involves a combination of clinical, radiological, and pathological findings, including the identification of specific markers.
## **Why the Correct Answer is Right**
The correct answer, **E. CD99 (MIC2)**, is a specific marker for Ewing's sarcoma because it is highly expressed in the majority of Ewing's sarcoma cases. CD99, also known as MIC2, is a cell surface glycoprotein that is encoded by the pseudoautosomal MIC2 gene. It is used as a diagnostic marker because it is consistently and strongly positive in Ewing's sarcoma, helping to differentiate it from other types of sarcomas and tumors.
## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain chromosomal translocations are associated with Ewing's sarcoma, such as t(11;22)(q24;q12) leading to the EWS-FLI1 fusion gene, this option is not specific enough as a marker compared to CD99.
- **Option B:** This option is not clearly defined but if it refers to other markers or genetic alterations, they are either not as specific or not commonly used as CD99 for Ewing's sarcoma.
- **Option C:** Similarly, without a specific marker or genetic alteration mentioned, it's hard to directly refute, but CD99 remains the most straightforward and specific marker listed.
- **Option D:** This option, like B and C, lacks specificity or direct relevance as a diagnostic marker for Ewing's sarcoma compared to CD99.
## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that while CD99 is highly sensitive and specific for Ewing's sarcoma, it can also be positive in other conditions, such as some lymphomas and synovial sarcomas, albeit usually with a different staining pattern. Therefore, the diagnosis of Ewing's sarcoma relies on a combination of clinical findings, radiographic features, and molecular studies, including the detection of the EWSR1 gene rearrangements.
## **Correct Answer:** . CD99 (MIC2)