**Core Concept**
Chronic myeloid leukemia (CML) is a clonal myeloproliferative disorder characterized by the uncontrolled proliferation of mature and immature myeloid cells. The disease is often associated with the presence of the Philadelphia chromosome, resulting from a reciprocal translocation between chromosomes 9 and 22, t(9;22)(q34;q11).

**Why the Correct Answer is Right**
The first-line treatment for CML involves targeted therapy with tyrosine kinase inhibitors (TKIs), which selectively inhibit the BCR-ABL tyrosine kinase. This enzyme is the product of the fusion gene resulting from the Philadelphia chromosome, and its activity drives the proliferation of leukemic cells. The most commonly used TKI in the treatment of CML is imatinib mesylate, which selectively inhibits the BCR-ABL tyrosine kinase, thereby reducing the proliferation of leukemic cells.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not represent a first-line treatment for CML.
**Option B:** This option is incorrect because it is not a targeted therapy for CML.
**Option D:** This option is incorrect because it is not a commonly used first-line treatment for CML.

**Clinical Pearl / High-Yield Fact**
It is essential for clinicians to recognize the importance of targeted therapy in the management of CML, as it has significantly improved patient outcomes. The development of TKIs has revolutionized the treatment of CML, making it a more manageable and chronic condition.

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