**Core Concept**
Familial hypercholesterolemia (FH) is a genetic disorder characterized by elevated levels of low-density lipoprotein (LDL) cholesterol, leading to premature coronary artery disease. It is caused by mutations in genes involved in the LDL receptor pathway, resulting in impaired clearance of LDL cholesterol from the bloodstream.

**Why the Correct Answer is Right**
The correct answer is related to the LDL receptor pathway. The LDL receptor plays a crucial role in removing LDL cholesterol from the bloodstream by binding to apolipoprotein B-100 on the surface of LDL particles. Defects in the LDL receptor gene can lead to impaired receptor function or reduced receptor expression, resulting in elevated LDL cholesterol levels. In FH, mutations in the LDLR gene, which encodes the LDL receptor, are the most common cause of the disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because mutations in the apolipoprotein B gene (APOB) can cause familial hypobetalipoproteinemia, a different disorder characterized by low levels of LDL and very-low-density lipoprotein (VLDL) cholesterol.

**Option B:** This option is incorrect because mutations in the PCSK9 gene, which encodes a protein involved in LDL receptor degradation, can cause familial hypercholesterolemia but are less common than LDLR mutations.

**Option C:** This option is incorrect because mutations in the APOE gene are associated with familial dysbetalipoproteinemia, a disorder characterized by elevated levels of both LDL and VLDL cholesterol.

**Option D:** This option is incorrect because mutations in the LPL gene are associated with familial chylomicronemia syndrome, a disorder characterized by elevated levels of triglyceride-rich lipoproteins.

**Clinical Pearl / High-Yield Fact**
FH is a treatable condition, and early diagnosis and treatment can significantly reduce the risk of cardiovascular disease. Patients with FH often present with premature atherosclerotic cardiovascular disease, and a family history of cardiovascular disease should raise suspicion for the disorder.

**Correct Answer:** A.