Dr. Kyriacou congratulates you on your response. Dr. Kyriacou is aware that you are currently preparing for taking the prescribing safety examination required for your postgraduate application and is confident that you will do well. 

Mr. Garcia is treated appropriately and discharged. He is advised not to consume grapefruit or its juice while taking simvastatin. You remember that grapefruit juice is a CYP450 inhibitor, which may prevent the break-down of simvastatin, leading to toxic levels. Thinking about the prescribing exam, you think it is a good idea to revise the mechanism of action of statins. 

What is the mechanism of action of these drugs?