**Core Concept**
Gastrointestinal stromal tumors (GISTs) are a type of mesenchymal tumor arising from the interstitial cells of Cajal or their precursors in the gastrointestinal tract. They are characterized by mutations in the KIT or PDGFRA genes, leading to uncontrolled cell proliferation.

**Why the Correct Answer is Right**
The marker specific for GIST is CD117, also known as c-KIT. This receptor tyrosine kinase is a product of the KIT gene, which is often mutated in GISTs. CD117 is expressed in the majority of GISTs and can be used as a diagnostic marker to distinguish GIST from other types of gastrointestinal tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** CD34 is a marker that is often co-expressed with CD117 in GISTs, but it is not specific for this tumor type. CD34 is also expressed in other types of mesenchymal tumors, such as leiomyomas and schwannomas.

**Option B:** S-100 protein is a marker for melanocytic tumors and is not specific for GIST. It is often used to diagnose malignant melanoma, but it can also be expressed in other types of tumors, such as schwannomas and granular cell tumors.

**Option C:** Vimentin is a marker for mesenchymal cells and is expressed in many types of tumors, including GIST. However, it is not specific for GIST and can be expressed in other types of tumors, such as leiomyomas and fibrosarcomas.

**Clinical Pearl / High-Yield Fact**
GISTs are the most common mesenchymal tumors of the gastrointestinal tract, and they can occur anywhere along the gastrointestinal tract, from the esophagus to the anus. They are often asymptomatic and are usually diagnosed incidentally on imaging studies.

**Correct Answer:** C. CD117.