Which of the following is known as a radiosensitizer?

5-Fluorouracil (5-FU) is recognized as a radiosensitizer because it enhances the effectiveness of radiation therapy. Radiosensitizers are agents that make cancer cells more susceptible to the damaging effects of radiation. In the case of 5-FU, it interferes with the cancer cell's ability to repair DNA damage caused by radiation, leading to an increase in cell death. This characteristic is particularly valuable in the treatment of certain types of cancers where combined modalities can produce better outcomes than either treatment alone.

The role of 5-FU in augmenting the effects of radiation has been studied in various cancers, particularly in gastrointestinal malignancies. Its mechanism involves the inhibition of thymidylate synthase, an enzyme crucial for DNA synthesis, thus making cancer cells more reliant on the mechanisms that radiation therapy exploits to cause harm.

In contrast, the other agents listed, while effective chemotherapy drugs, do not possess the same radiosensitizing properties that enhance radiation effects in a clinically significant manner. Understanding the role of radiosensitizers like 5-FU allows oncology professionals to optimize treatment regimens and improve patient outcomes.