**Question:** Imatinib mesylate used in the treatment of GIST is a inhibitor of

A. Abl tyrosine kinase
B. Bcr-Abl tyrosine kinase
C. c-Kit tyrosine kinase
D. PDGFR tyrosine kinase

**Correct Answer:** C. c-Kit tyrosine kinase

**Core Concept:** Imatinib mesylate is a tyrosine kinase inhibitor used in the treatment of gastrointestinal stromal tumors (GIST). Tyrosine kinases are enzymes that play a crucial role in various cellular processes, including cell growth, differentiation, and survival.

**Why the Correct Answer is Right:** Imatinib mesylate is an inhibitor of c-Kit tyrosine kinase, which is a receptor protein tyrosine kinase involved in the signaling pathways responsible for cell growth and differentiation. In GIST, c-Kit mutations lead to uncontrolled cell proliferation, and imatinib mesylate targets these mutated c-Kit proteins, inhibiting their activity and halting tumor growth.

**Why Each Wrong Option is Incorrect:**

A. Abl tyrosine kinase (Option A) is involved in chronic myeloid leukemia (CML) and is not the primary target in GIST. Imatinib mesylate is primarily used for treating CML, not GIST.

B. Bcr-Abl tyrosine kinase (Option B) is involved in chronic myeloid leukemia (CML), not GIST. Imatinib mesylate is primarily used for treating CML, not GIST.

C. c-Kit tyrosine kinase (Correct Option) is a receptor protein tyrosine kinase involved in cell signaling pathways in GIST. Imatinib mesylate targets the mutated c-Kit proteins in GIST, leading to its therapeutic efficacy.

D. PDGFR tyrosine kinase (Option D) is involved in platelet-derived growth factor receptor signaling pathways. Imatinib mesylate can inhibit PDGFR, but its primary target in GIST is c-Kit tyrosine kinase, which is why Option D is incorrect.

**Clinical Pearl:** Imatinib mesylate's primary target is c-Kit tyrosine kinase, which is overexpressed or mutated in a significant proportion of GIST cases. By inhibiting c-Kit, imatinib mesylate helps to halt tumor growth and improve the prognosis for patients with GIST.