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

**Why the Correct Answer is Right**
GISTs are generally treated with targeted therapy, specifically tyrosine kinase inhibitors (TKIs) such as imatinib, which blocks the KIT and PDGFRA tyrosine kinases. This leads to inhibition of tumor growth and regression of existing tumors. The effectiveness of TKIs in GISTs is a testament to the importance of molecular targeted therapy in cancer treatment.

**Why Each Wrong Option is Incorrect**
**Option A:** GISTs are typically slow-growing and benign. This is incorrect because while some GISTs may be benign, many others are malignant and can exhibit aggressive behavior.

**Option B:** Imatinib is the first-line treatment for GISTs. This is correct, but it's the correct answer, so we'll move on.

**Option C:** GISTs are more common in the stomach than in the small intestine. This is incorrect because while GISTs can occur in both the stomach and small intestine, they are more commonly found in the small intestine.

**Option D:** GISTs are typically diagnosed with imaging studies and biopsy. This is correct, but again, it's the correct answer.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that GISTs can occur anywhere in the gastrointestinal tract, from the esophagus to the rectum. A high index of suspicion is necessary for early diagnosis and treatment.

**Correct Answer: A. GISTs are typically slow-growing and benign.**