Review Question - QID 109889

An 18-month-old boy is brought to his pediatrician because his mother is concerned about his walking. The boy was born at 37 weeks’ gestation and spent 1 night in the neonatal intensive care unit for respiratory distress. He otherwise has been healthy and started walking independently at 12 months. However, his mother reports that he has stopped walking and appears weaker than he was a few months ago. He also has started having brief convulsive episodes over the past week. His parents are both healthy with no medical conditions. On exam, the child demonstrates muscle rigidity and diffuse muscle wasting. He is unable to stand or sit up straight. His patellar, Achilles, and brachioradialis reflexes are absent. Fundoscopic examination is unremarkable. A urine sample demonstrates metachromic granules. This patient’s condition is caused by a defect in which of the following enzymes?