SBA — Single Best Answer · Review
Question 446
Pharmacology → General
Classification
Question
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A drug whose clearance depends substantially on hepatic microsomal metabolism is given with a second agent. Which pairing would be expected to exert opposing effects on hepatic microsomal enzyme activity?
Question Stem
Select the single best answer.
Options
A
Phenytoin and carbamazepine
B
Rifampicin and phenobarbitone
C
Erythromycin and fluconazole
D
Carbamazepine and erythromycin
E
Cimetidine and ketoconazole
Explanation
Carbamazepine induces hepatic microsomal enzymes, whereas erythromycin inhibits them. Co-administration therefore introduces opposing influences on microsomal drug metabolism. Phenytoin, rifampicin and phenobarbitone are also enzyme inducers, whereas fluconazole, cimetidine and ketoconazole are enzyme inhibitors.
Option Validity
A) Both phenytoin and carbamazepine are hepatic microsomal enzyme inducers.
B) Both rifampicin and phenobarbitone are enzyme inducers.
C) Erythromycin and fluconazole are both listed as enzyme inhibitors.
E) Cimetidine and ketoconazole are both listed as enzyme inhibitors.
Further Reading
Saxena R. Textbook for MRCOG-1. Chapter 12: Pharmacology. Clinical Pharmacy, pp.344–345.