**Question:** Drug of choice for treatment of chronic myeloid leukemia is

A. Imatinib
B. Hydroxyurea
C. Interferon-alpha
D. Busulfan

**Correct Answer:** D. Busulfan

**Core Concept:**
Chronic myeloid leukemia (CML) is a type of cancer affecting white blood cells, specifically the myeloid lineage. The disease is characterized by the presence of the Philadelphia chromosome, a chromosomal abnormality that results from the fusion of the ABL and BCR genes. This fusion gene, called BCR-ABL, encodes a constitutively active tyrosine kinase enzyme, which drives the proliferation of myeloid cells and leads to the development of CML.

**Why the Correct Answer is Right:**
Busulfan is a chemotherapeutic agent that primarily acts by inhibiting DNA synthesis and cell division. In the context of CML, busulfan's mechanism of action is particularly relevant as it targets the excessively dividing myeloid cells caused by the BCR-ABL fusion gene. This drug effectively suppresses the proliferation of these cells and helps in achieving remission in patients with chronic myeloid leukemia.

**Why Each Wrong Option is Incorrect:**

A. Imatinib: Imatinib (Gleevec) is a tyrosine kinase inhibitor that specifically targets the BCR-ABL fusion protein. While it is an effective therapy for CML, busulfan's mechanism of action is more broad and does not specifically target the BCR-ABL fusion gene.

B. Hydroxyurea: Hydroxyurea is a cytotoxic agent that interferes with DNA synthesis. Its mechanism is similar to busulfan, as it inhibits DNA synthesis. However, it does not specifically target the BCR-ABL fusion gene or the CML disease process.

C. Interferon-alpha: Interferon-alpha is an immunomodulatory agent that has antiviral and antiproliferative properties. While it may have some clinical benefits in CML treatment, its mechanism is not specifically directed against the BCR-ABL fusion gene or the disease process.

**Clinical Pearl:**
Busulfan is a chemotherapeutic agent with a broader mechanism of action, targeting DNA synthesis in rapidly dividing cells. In the context of chronic myeloid leukemia, it helps in controlling the disease process by suppressing the excessive proliferation of myeloid cells but does not specifically target the BCR-ABL fusion gene like Imatinib does. Busulfan is commonly used in combination with other agents, such as interferon-alpha, to achieve better treatment outcomes in patients with chronic myeloid leukemia.