**Core Concept**
The question is testing the understanding of the mechanism of action of statins, a class of lipid-lowering medications. HMG-CoA reductase is a key enzyme in the biosynthesis of cholesterol, and its inhibition is the primary mechanism by which statins lower cholesterol levels.

**Why the Correct Answer is Right**
Lovastatin, a statin, works by directly inhibiting the expression of the HMG-CoA reductase gene. This inhibition is achieved through the binding of lovastatin to the HMG-CoA reductase enzyme, which prevents the enzyme from being transcribed and translated into a functional protein. As a result, the synthesis of cholesterol is reduced, leading to decreased levels of low-density lipoprotein (LDL) cholesterol in the blood.

**Why Each Wrong Option is Incorrect**
**Option A:** Ezetimibe inhibits the absorption of cholesterol in the small intestine, but it does not directly inhibit the expression of the HMG-CoA reductase gene.
**Option B:** Fibrates work by activating peroxisome proliferator-activated receptor-alpha (PPAR-alpha), which increases the expression of lipoprotein lipase and decreases the production of triglycerides. However, they do not directly inhibit the expression of the HMG-CoA reductase gene.
**Option C:** Nicotinic acid (niacin) increases the expression of high-density lipoprotein (HDL) cholesterol and decreases the production of very-low-density lipoprotein (VLDL) cholesterol, but it does not directly inhibit the expression of the HMG-CoA reductase gene.

**Clinical Pearl / High-Yield Fact**
Statins are a class of medications that can be used to lower cholesterol levels in patients with hyperlipidemia. They are often used in combination with other lipid-lowering therapies, such as ezetimibe and fibrates, to achieve optimal lipid profiles.

**Correct Answer: C. Lovastatin**