Q. A 7-month-old boy is brought to the pediatrician's office for a routine examination. He is feeding well, achieving appropriate developmental milestones, and has no complaints. The parents report no episodes of jaundice, vomiting, diarrhea, or any other symptoms. As part of the examination, the physician notes reducing substances in both serum and urine samples but no other abnormalities. Which of the following enzymes is deficient in this patient?