**Core Concept**
Gastrointestinal stromal tumors (GISTs) are a type of mesenchymal tumor originating from the interstitial cells of Cajal or their precursors. The treatment of GISTs involves targeted therapy, and the choice of agent depends on the molecular characteristics of the tumor.

**Why the Correct Answer is Right**
Imatinib mesylate is a tyrosine kinase inhibitor that targets the KIT and PDGFRA receptors. GISTs often harbor mutations in the KIT or PDGFRA genes, leading to constitutive activation of these receptors. Imatinib selectively inhibits the ATP-binding site of these receptors, thereby blocking the downstream signaling pathways that promote tumor growth and survival. This mechanism of action makes imatinib an effective treatment for GISTs.

**Why Each Wrong Option is Incorrect**
**Option A:** Sunitinib is a multitargeted tyrosine kinase inhibitor that is used as a second-line treatment for GISTs. It has a broader spectrum of activity than imatinib but is not as effective in the first-line setting.
**Option B:** Sorafenib is another multitargeted tyrosine kinase inhibitor that is used in various cancers, including renal cell carcinoma and hepatocellular carcinoma. It is not a standard treatment for GISTs.
**Option C:** Bevacizumab is a monoclonal antibody that targets vascular endothelial growth factor (VEGF). It is used in the treatment of various cancers, including colorectal and breast cancer, but not typically for GISTs.

**Clinical Pearl / High-Yield Fact**
Imatinib is a classic example of a targeted therapy that has revolutionized the treatment of GISTs. The response to imatinib is often dramatic, with significant tumor shrinkage and improved survival outcomes.

**Correct Answer:** C. Imatinib mesylate.