**Core Concept**
The question revolves around the concept of radiation toxicity and the need for dose adjustment of certain drugs during radiotherapy to prevent or minimize its adverse effects. This involves understanding the pharmacological interactions between chemotherapeutic agents and radiation therapy.

**Why the Correct Answer is Right**
The correct answer is a drug that requires dose adjustment during radiotherapy to prevent radiation toxicity. This is because the drug interacts with radiation, either by enhancing its effects or by causing increased toxicity when given together. For instance, some drugs may increase the risk of radiation-induced mucositis, dermatitis, or other toxicities. The correct answer is a drug that inhibits the enzyme dihydropyrimidine dehydrogenase (DPD), which is involved in the catabolism of 5-fluorouracil (5-FU), a chemotherapeutic agent commonly used in conjunction with radiation therapy. Inhibiting DPD leads to increased levels of 5-FU, enhancing its antitumor effects but also increasing the risk of radiation toxicity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the question as it does not interact with radiation therapy in a way that would require dose adjustment.
* **Option B:** This option is incorrect because it does not have a known interaction with radiation therapy that would necessitate dose adjustment.
* **Option C:** This option is not the correct answer as it is not known to interact with radiation therapy in a way that would require dose adjustment.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that certain drugs, particularly those that inhibit DPD, require dose adjustment during radiotherapy to prevent radiation toxicity. This is crucial in the management of patients undergoing concurrent chemoradiation therapy.

**Correct Answer: C. Capecitabine**