**Core Concept**
The question is testing the understanding of the cell cycle-specificity of various anti-cancer drugs, which is crucial in cancer chemotherapy. **Cell cycle-specific** drugs target rapidly dividing cells, such as cancer cells, at specific phases of the cell cycle, whereas **cell cycle-nonspecific** drugs can target cells in any phase of the cell cycle.

**Why the Correct Answer is Right**
Ifosfamide is an alkylating agent that works by cross-linking DNA strands, thereby inhibiting DNA replication and transcription in all phases of the cell cycle, including the S phase. This makes it a **cell cycle-nonspecific** drug, unlike the other options which are all **S-phase specific**. S-phase specific drugs target cells during the synthesis phase of the cell cycle when DNA replication occurs.

**Why Each Wrong Option is Incorrect**
**Option A:** Methotrexate is an antifolate that inhibits dihydrofolate reductase (DHFR), an enzyme required for DNA synthesis, making it **S-phase specific**.
**Option B:** Mercaptopurine is a purine analog that interferes with DNA synthesis, making it **S-phase specific**.
**Option D:** Thioguanine is also a purine analog that inhibits DNA synthesis, making it **S-phase specific**.

**Clinical Pearl / High-Yield Fact**
When choosing an anti-cancer drug, it's essential to consider the cell cycle specificity of the drug to minimize damage to normal cells and maximize efficacy against cancer cells.

**✓ Correct Answer: C. Ifosfamide**