A 32-year-old woman presents with progressive yellow-orange papules and nodules on her elbows, knees, and buttocks over 5 years. Examination also reveals yellow plaques on her eyelids and corneal arcus despite her young age. Her father died of myocardial infarction at 38 years, and her paternal uncle at 42 years. Fasting lipid profile shows total cholesterol 480 mg/dL, LDL 380 mg/dL, HDL 42 mg/dL, and triglycerides 180 mg/dL. The skin and tendon lesions marked **A** in the diagram are lipid-laden macrophage deposits. Which of the following genetic mutations is MOST LIKELY responsible for her presentation?