**Core Concept**
Hairy cell leukemia is a rare, chronic cancer of the blood characterized by the accumulation of abnormal B lymphocytes. The disease is often treated with chemotherapy and immunotherapy, with the goal of eradicating cancer cells while preserving normal hematopoiesis.

**Why the Correct Answer is Right**
Cladribine, a nucleoside analog, is the drug of choice for hairy cell leukemia due to its high efficacy in inducing complete remission. Cladribine works by incorporating into DNA and interfering with DNA synthesis, thereby inhibiting cancer cell proliferation. This mechanism of action is particularly effective against hairy cell leukemia cells, which exhibit a high rate of cell turnover. Cladribine has also been shown to induce long-term remissions, with some patients remaining in remission for many years after treatment.

**Why Each Wrong Option is Incorrect**
* **Option A:** Methotrexate is a folate antagonist used to treat a variety of cancers, including leukemia. However, it is not the preferred treatment for hairy cell leukemia due to its lower efficacy and higher toxicity compared to cladribine.
* **Option B:** Gemcitabine is a nucleoside analog used to treat pancreatic cancer, breast cancer, and other solid tumors. While it has some activity against hematologic malignancies, it is not the first-line treatment for hairy cell leukemia.
* **Option D:** Decitabine is a hypomethylating agent used to treat myelodysplastic syndromes and acute myeloid leukemia. It has some activity against hairy cell leukemia, but it is not as effective as cladribine in inducing complete remissions.

**Clinical Pearl / High-Yield Fact**
The use of cladribine for hairy cell leukemia is a classic example of targeted therapy, where the treatment is tailored to the specific molecular characteristics of the cancer cells. Cladribine's high efficacy and relatively low toxicity make it a preferred treatment option for this disease.

✓ Correct Answer: C. Cladribine