**Core Concept**
Radiosensitisers are a class of drugs that enhance the sensitivity of cancer cells to radiation therapy, thereby increasing the effectiveness of the treatment. They typically work by damaging the DNA of cancer cells, making them more susceptible to radiation-induced damage.

**Why the Correct Answer is Right**
Amifostine is a cytoprotective agent, not a radiosensitiser. It works by protecting normal tissues from the toxic effects of radiation and chemotherapy, rather than enhancing the sensitivity of cancer cells to radiation. Specifically, amifostine inhibits the enzyme cytidine deaminase, which is involved in the metabolism of certain chemotherapeutic agents. This action helps to reduce the side effects of chemotherapy and radiation therapy on normal tissues.

**Why Each Wrong Option is Incorrect**
**Option A:** Metronidazole is a radiosensitiser that works by interfering with the repair of DNA damage caused by radiation. It is commonly used in combination with radiation therapy to treat certain types of cancer, such as cervical cancer.

**Option B:** Nimorazole is another radiosensitiser that has been shown to enhance the effectiveness of radiation therapy in treating certain types of cancer, such as head and neck cancer. It works by damaging the DNA of cancer cells, making them more susceptible to radiation-induced damage.

**Option C:** Cisplatin is a chemotherapeutic agent that is also a radiosensitiser. It works by damaging the DNA of cancer cells, making them more susceptible to radiation-induced damage. Cisplatin is commonly used in combination with radiation therapy to treat certain types of cancer, such as testicular cancer.

**Clinical Pearl / High-Yield Fact**
Radiosensitisers can be broadly classified into two categories: direct and indirect radiosensitisers. Direct radiosensitisers, such as metronidazole and nimorazole, work by damaging the DNA of cancer cells directly, while indirect radiosensitisers, such as cisplatin, work by enhancing the effects of radiation on cancer cells.

**✓ Correct Answer: D. Amifostine. Amifostine is a cytoprotective agent that protects normal tissues from the toxic effects of radiation and chemotherapy.**