A 9-year-old boy has progressive night blindness, anosmia, ataxia, and scaly skin. Fundus examination shows retinal pigmentary changes. Serum total cholesterol is 120 mg/dL with a normal triglyceride level. Urine analysis detects phytanic acid. The underlying enzymatic defect is:

• A Sterol 27-hydroxylase
• B Mitochondrial very long chain acyl-CoA dehydrogenase
• C Microsomal triglyceride transfer protein
• D Peroxisomal phytanoyl-CoA alpha-hydroxylase ✓

Explanation

Refsum disease results from defective alpha-oxidation of the branched-chain fatty acid phytanic acid due to phytanoyl-CoA hydroxylase deficiency, causing its accumulation with neurologic, retinal, and cutaneous features. VLCFA are normal here, unlike peroxisome biogenesis disorders, and MTP defects cause abetalipoproteinemia with fat malabsorption and absent chylomicrons. Sterol 27-hydroxylase deficiency causes cerebrotendinous xanthomatosis.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.