Phenytoin affects the metabolism of other drugs that utilize which enzyme system?

• A. Hepatic microsomal cytochrome P450 enzyme system
• B. Renal glomerular filtration
• C. Intestinal transporters
• D. Brain monoamine oxidase

Answer: (A)

Phenytoin is a potent inducer of the hepatic microsomal cytochrome P450 enzyme system. By boosting the activity of these liver enzymes, drugs that are substrates of this system are metabolized faster, lowering their plasma concentrations and potentially reducing efficacy or necessitating dose adjustments. This mechanism explains why phenytoin has many drug interactions. The other options don’t fit because renal glomerular filtration concerns elimination by the kidneys, intestinal transporters affect absorption, and brain monoamine oxidase is a different enzyme with a separate role, not the primary pathway for drug metabolism in this context.