This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
Which of the following drugs is an inhibitor of cytochrome p450 enzymes ? 
Select and explain. 
A. Ketoconazole, B. Rifampicin, C. Phenytoin, D. Phenobarbitone
A: Ketoconazole
Answer: A: Ketoconazole 
Explanation: - Cytochrome p450 enzymes are microsomal enzymes (present on smooth endoplasmic reticulum).
- Of the given options, only Ketoconazole is a microsomal enzyme inhibitor. Rest all are enzyme inducers.