What does phase I metabolism generally involve?

Phase I metabolism primarily involves chemical modifications of drugs or their metabolites through reactions such as oxidation, reduction, or hydrolysis. This phase is crucial for making lipophilic compounds more hydrophilic, which typically enhances their excretion in urine. During Phase I, enzymes often introduce functional groups or expose existing ones to facilitate further modification in Phase II.

In contrast, the other options touch upon aspects of metabolism but do not accurately describe the main characteristics of Phase I. The synthesis of drug metabolites for urine pertains more to the overall pharmacokinetic process rather than the specific biochemical reactions of Phase I. Conjugation reactions, which involve linking drugs with larger molecules to increase their water solubility, are distinct to Phase II metabolism. Lastly, direct excretion of unchanged drugs is a process that can happen outside of metabolism and typically refers to how some drugs are eliminated without undergoing significant change.