Q. A 6-month-old infant is brought to the pediatric clinic for a routine checkup. The mother is concerned about the infant's skin and eye color, which differ from those of her, the father, and their older child. She reports that the baby's skin is significantly lighter, and her eyes are lighter blue compared to the rest of the family, who all have brown eyes and skin of a darker complexion. On examination, the child's skin and hair lack pigmentation. The rest of the physical examination is unremarkable. The child's clinical presentation is suggestive of a condition resulting from a deficiency in which of the following enzymes?