**Core Concept**
Amifostine is a cytoprotective agent used to reduce the side effects of chemotherapy and radiation therapy. It works by protecting the normal cells from the toxic effects of these treatments, thereby reducing the risk of complications.

**Why the Correct Answer is Right**
Amifostine acts as a radioprotector by scavenging free radicals and reducing the formation of lipid peroxides, which can cause damage to normal cells. It also inhibits the enzyme topoisomerase II, which is involved in DNA replication, thereby reducing the risk of DNA damage. Amifostine is administered intravenously or subcutaneously and is most commonly used to reduce the risk of xerostomia (dry mouth) in patients undergoing radiation therapy for head and neck cancer.

**Why Each Wrong Option is Incorrect**
**Option B:** Cisplatin is a chemotherapeutic agent that works by forming platinum-DNA adducts, which interfere with DNA replication and transcription. It does not have radioprotective effects.
**Option C:** Mesna is a chemoprotective agent that is used to prevent hemorrhagic cystitis caused by ifosfamide and cyclophosphamide. It does not have radioprotective effects.
**Option D:** Tirapazamine is a chemotherapeutic agent that is used to treat certain types of cancer, including cervical and head and neck cancer. It works by generating free radicals that damage DNA, and does not have radioprotective effects.

**Clinical Pearl / High-Yield Fact**
Amifostine is the only radioprotector drug that is currently approved for clinical use. It is an important adjunct to radiation therapy, particularly in patients undergoing treatment for head and neck cancer.

✓ Correct Answer: A. Amifostine