**Core Concept**
Decitabine is a chemotherapeutic agent that exerts its antineoplastic effects through the process of DNA hypomethylation. This involves the inhibition of DNA methyltransferase enzymes, leading to a decrease in the methylation of cytosine residues in CpG islands. As a result, silenced tumor suppressor genes are reactivated, contributing to the anticancer activity of decitabine.

**Why the Correct Answer is Right**
Decitabine is a nucleoside analog that is specifically designed to mimic the structure of the DNA building block cytidine. Upon incorporation into DNA, decitabine acts as a potent inhibitor of DNA methyltransferases (DNMTs), the enzymes responsible for the addition of methyl groups to cytosine residues. By preventing the methylation of CpG islands, decitabine allows for the reactivation of silenced tumor suppressor genes, thereby exerting its anticancer effects. This mechanism is distinct from other chemotherapeutic agents that act through DNA cross-linking, topoisomerase inhibition, or microtubule disruption.

**Why Each Wrong Option is Incorrect**
**Option A:** Gemcitabine is a nucleoside analog that acts as a false substrate for DNA polymerase, leading to DNA chain termination and apoptosis in rapidly dividing cancer cells. It does not exert its effects through DNA hypomethylation.
**Option B:** 5-Fluorouracil (5-FU) is a thymidylate synthase inhibitor that acts by disrupting DNA synthesis and repair, leading to cell death in cancer cells. It does not involve DNA hypomethylation in its mechanism of action.
**Option D:** Homoharringtonine is a plant alkaloid that acts as a protein synthesis inhibitor, preventing the translation of mRNA into protein and leading to cell death in cancer cells. It does not involve DNA hypomethylation in its mechanism of action.

**Clinical Pearl / High-Yield Fact**
Decitabine is often used in the treatment of myelodysplastic syndromes (MDS) and acute myeloid leukemia (AML), where it has been shown to improve survival and reduce the risk of disease progression. Its ability to reactivate silenced tumor suppressor genes makes it a valuable addition to the chemotherapy armamentarium for these diseases.

✓ Correct Answer: C. Decitabine