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Question 436
Pharmacology → General
Classification
Question
SSBAQuestion Header
A drug undergoes hepatic metabolism. Its first reaction is cytochrome P450-mediated oxidation in the endoplasmic reticulum, followed by conjugation with glucuronide in the cytoplasm. Which description correctly identifies these reactions?
Question Stem
Select the single best answer.
Options
A
Oxidation is Phase II and glucuronidation is Phase I
B
Both reactions are Phase I
C
Oxidation is Phase I and glucuronidation is Phase II
D
Both reactions are Phase II
E
Neither reaction is involved in increasing drug polarity
Explanation
Phase I reactions occur in the endoplasmic reticulum and include cytochrome P450-mediated oxidation, hydrolysis and reduction, producing more polar metabolites. Phase II reactions occur in the cytoplasm and involve conjugation with compounds such as glucuronides, sulphates, glutathione or amino acids, facilitating formation of more readily excretable metabolites.
Option Validity
A) The sequence is reversed; oxidation is a Phase I reaction whereas glucuronide conjugation is Phase II.
B) Glucuronidation is a conjugation reaction and therefore belongs to Phase II metabolism.
D) Cytochrome P450-mediated oxidation is characteristic of Phase I metabolism.
E) Drug metabolism commonly increases polarity and thereby facilitates excretion.
Further Reading
Saxena R. Textbook for MRCOG-1. Chapter 12: Pharmacology. Clinical Pharmacy, p.344.