**Core Concept**
Xenobiotics are foreign substances that enter the body, and their metabolism is a crucial process to convert them into less toxic or more water-soluble compounds, facilitating their excretion. This process is primarily carried out by the cytochrome P450 enzyme system in the liver.

**Why the Correct Answer is Right**
Xenobiotics undergo phase I metabolism, which involves the conversion of lipophilic compounds into more hydrophilic ones through reactions such as oxidation, reduction, or hydrolysis. This process is primarily mediated by the cytochrome P450 enzyme system, which contains various isoforms (e.g., CYP1A2, CYP2D6, CYP3A4) that catalyze these reactions. The resulting metabolites are then further conjugated in phase II metabolism to increase their water solubility.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it doesn't specify the type of metabolism or the enzyme system involved in xenobiotic metabolism.
**Option B:** Incorrect because it's a vague term that could refer to various types of metabolism, not specifically xenobiotic metabolism.
**Option C:** Incorrect because it's not a recognized type of xenobiotic metabolism.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the cytochrome P450 enzyme system is highly polymorphic, leading to varying metabolic rates among individuals. This can result in different responses to drugs and increased susceptibility to adverse effects.

**Correct Answer: D. Phase I metabolism via cytochrome P450 enzymes**