A 6-year-old boy is brought to your pediatric clinic by his parents who are concerned with his growth and development. They immigrated to the United States a few months ago from Tunisia. He has not been growing as he should compared to other children of his age. At school, he has learning disabilities. His reading skills are poor. His motor skills are inadequate. Other children make fun of him for having a ‘musty body odor’. His behavior becomes erratic when he eats certain foods like beef, pork, lamb, venison, salmon, trout and drinks diet sodas. On physical examination, the boy has blue eyes and blonde hair; microcephaly, hypertonia and hyperreflexia in all four limbs. Biochemical defect in this disorder is in which of the following proteins?
