**Core Concept**
The question revolves around the regulation of cholesterol levels in the body, specifically the LDL cholesterol pathway. The LDL receptor plays a crucial role in removing excess cholesterol from the bloodstream by binding to LDL particles and facilitating their internalization by cells. Abnormalities in this pathway can lead to elevated LDL cholesterol levels.

**Why the Correct Answer is Right**
In this scenario, the patient has normal levels of LDL receptors, which rules out receptor-mediated defects as the cause of high LDL levels. The most probable cause is an increase in the activity of HMG-CoA reductase, the rate-limiting enzyme in the cholesterol biosynthesis pathway. This enzyme catalyzes the conversion of HMG-CoA to mevalonate, a crucial step in the synthesis of cholesterol. Increased HMG-CoA reductase activity leads to an overproduction of cholesterol, resulting in elevated LDL levels.

**Why Each Wrong Option is Incorrect**

**Option A:** This option is not relevant to the scenario, as it does not directly relate to the regulation of cholesterol levels or the function of LDL receptors.

**Option B:** This option is incorrect because it does not address the normal levels of LDL receptors in the patient. Defects in LDL receptor function or number would be expected to cause elevated LDL levels, but this is not the case here.

**Option C:** This option is incorrect because it does not explain the underlying mechanism of elevated LDL levels in the patient. While genetic disorders can cause elevated LDL levels, they are not the most probable cause in this scenario given the normal levels of LDL receptors.

**Clinical Pearl / High-Yield Fact**
The HMG-CoA reductase inhibitors, also known as statins, are a class of medications that work by inhibiting the activity of HMG-CoA reductase, thereby reducing cholesterol production in the liver. They are commonly used to treat high cholesterol and have been shown to reduce the risk of cardiovascular disease.

**Correct Answer:** C. Increased activity of HMG-CoA reductase.