## **Core Concept**
Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal neoplasms of the gastrointestinal tract. They originate from the interstitial cells of Cajal or their precursors. The core concept here involves identifying a specific molecular marker that is closely associated with GISTs.

## **Why the Correct Answer is Right**
The correct answer, **CD117 (c-KIT)**, is right because GISTs frequently express the **c-KIT** protein, which is a receptor tyrosine kinase. Approximately 70-80% of GISTs have mutations in the **KIT** gene, and about 5-10% have mutations in the **PDGFRA** gene. The c-KIT protein is a key marker for the diagnosis of GISTs, and its detection by immunohistochemistry is a critical diagnostic tool.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **CD34** is a marker that can be positive in GISTs, it is not as specific or sensitive as **CD117 (c-KIT)**. CD34 is also positive in other types of tumors.
- **Option B:** **S-100** is a marker commonly used for neural crest-derived tumors, such as melanomas and schwannomas, and is not specific for GISTs.
- **Option D:** **CD10** is not typically used as a marker for GISTs; it is more commonly associated with certain types of lymphomas and carcinomas.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **CD117 (c-KIT)** positivity is a hallmark for GISTs, but it's worth noting that not all GISTs are **CD117 positive**. A subset of GISTs, particularly those with **PDGFRA** mutations, may be **CD117 negative**. Therefore, a comprehensive diagnostic approach including molecular studies may be necessary in some cases.

## **Correct Answer:** . CD117 (c-KIT)