Phase I Metabolism — FRCA Primary MCQ
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Correct answer: C — Cytochrome P450-mediated oxidation
Explanation lettering: D = shown as A · E = shown as C · C = shown as D · A = shown as E
Cytochrome P450-mediated oxidation is characteristically a phase I reaction. Phase I metabolism comprises functionalisation reactions—principally oxidation, reduction and hydrolysis—which introduce or expose functional groups and may prepare a drug for further metabolism. Glutathione conjugation, glucuronidation, acetylation and sulphation are phase II reactions: they conjugate the drug or metabolite with an endogenous group, usually facilitating elimination. Phase II does not invariably follow phase I; a parent drug that already possesses a suitable functional group may undergo conjugation directly. Therefore, E is the single best answer, while A–D are all examples of phase II metabolism.
Reference: Abdelmonem BH et al. Decoding the Role of CYP450 Enzymes in Metabolism and Disease: A Comprehensive Review. Biomedicines. 2024;12(7):1467. https://pubmed.ncbi.nlm.nih.gov/16472997/