Lipid Biochemistry USMLE Step 1 Practice Question

A 3-year-old boy with consanguineous parents presents with failure to thrive, developmental delay, and progressive ataxia with nystagmus. Vital signs show HR 92/min, RR 20/min, BP 95/60 mmHg, temp 37°C. Serum cholestanol is markedly elevated at 18 mg/dL (normal <1 mg/dL) with abnormal cholesterol metabolism. MRI shows cerebellar atrophy. He lacks xanthomas or peripheral neuropathy. Which enzyme is most likely deficient?