Q. A 1-year-old boy is brought to the clinic by concerned parents who have noticed he is not meeting his developmental milestones. They mention that he initially seemed to be developing normally but has recently shown regression in motor skills. On examination, the child displays gait difficulties, hypotonia, and extensor plantar responses. An ophthalmic exam reveals optic atrophy. Nerve conduction studies indicate marked slowing. Brain MRI demonstrates symmetric white matter lesions with a periventricular and/or frontal predominance. Which of the following enzymes is deficient in this patient?