**Core Concept**
Gastrointestinal stromal tumors (GISTs) are a type of mesenchymal tumor that arises from the interstitial cells of Cajal or their precursors in the gastrointestinal tract. They are characterized by the presence of specific genetic mutations, including KIT and PDGFRA gene rearrangements.

**Why the Correct Answer is Right**
The most specific marker for GIST is CD117, also known as c-KIT. This is because GISTs often express high levels of CD117, which is a receptor tyrosine kinase that is involved in the signaling pathway of stem cell factor (SCF). The CD117 protein is produced as a result of the KIT gene rearrangement or mutation that is common in GISTs. Immunohistochemical staining for CD117 is used as a diagnostic marker to differentiate GISTs from other types of gastrointestinal tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** Cytokeratins are a family of intermediate filament proteins that are expressed in epithelial cells, making them a specific marker for epithelial tumors, not GISTs.
**Option B:** S-100 protein is a marker for neural crest-derived tumors, such as melanomas and schwannomas, but not GISTs.
**Option C:** Vimentin is a intermediate filament protein that is expressed in a wide range of tumor types, including sarcomas, carcinomas, and melanomas, making it a non-specific marker for GISTs.

**Clinical Pearl / High-Yield Fact**
When diagnosing GISTs, it's essential to remember that CD117 expression is not exclusive to GISTs, and some GISTs may be negative for CD117. Therefore, a combination of clinical, radiological, and histopathological features should be considered in conjunction with CD117 immunohistochemical staining to establish a definitive diagnosis.

**Correct Answer:** C. CD117