This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Hematology and CVS, Subject name: Pharmacology
Mechanism of action of atorvastatin is:- 
Select and explain. 
A. HMG-CoA synthase stimulator, B. HMG-CoA reductase stimulator, C. HMG-CoA synthase inhibitor, D. HMG-CoA reductase inhibitor
D: HMG-CoA reductase inhibitor
Answer: D: HMG-CoA reductase inhibitor 
Explanation: HMG CoA reductase catalyses the rate limiting step in cholesterol biosynthesis (conversion of HMG CoA to mevalonate). Statins act by inhibiting this enzyme competitively and result in decreased cholesterol synthesis in the liver. As liver requires cholesterol for synthesis of bile acids and steroid hormones, it responds by increasing the uptake of LDL from the plasma. This is done by increasing LDL receptors on its surface. Statins are most powerful LDL lowering agents.