**Core Concept**
Radiation toxicity is a major concern in cancer treatment, and certain medications can exacerbate this condition. Drugs that affect the metabolism of radiation or interact with the body's response to radiation can require dose adjustments to prevent toxicity.

**Why the Correct Answer is Right**
The correct answer involves a drug that is metabolized by the enzyme CYP2D6, which is also a major enzyme responsible for the metabolism of many other drugs. Radiation therapy can induce CYP2D6, leading to increased metabolism of certain drugs and potentially reduced efficacy or increased toxicity. One such drug is **Metoprolol**, a beta-blocker used to treat hypertension and heart rate control. Metoprolol is metabolized by CYP2D6 and requires dose adjustment during radiotherapy to prevent toxicity.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not involve a drug that requires dose adjustment during radiotherapy.
* **Option B:** This option is incorrect because it does not involve a drug that is metabolized by CYP2D6.
* **Option C:** This option is incorrect because it involves a drug that does not require dose adjustment during radiotherapy.

**Clinical Pearl / High-Yield Fact**
When managing patients undergoing radiotherapy, it is essential to consider the potential interactions between radiation and medications. Drugs that are metabolized by CYP2D6, such as metoprolol, may require dose adjustments to prevent toxicity.

**Correct Answer:** C. Metoprolol.