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 is an enzyme inhibitor ? 
Select and explain. 
A. Ketoconazole, B. Rifampicin, C. Tolbutamide, D. Phenobarbitone
A: Ketoconazole
Answer: A: Ketoconazole 
Explanation: None