Q. A newborn baby girl is brought to the pediatrician by her mother. She expresses worries about the baby being excessively floppy, having difficulties feeding, and not gaining weight as expected. On physical examination, the pediatrician also notes a distinct musty body odor, hypopigmented skin, and eczema. After extensive workup, these clinical findings point towards a specific metabolic disorder. Which enzyme deficiency is most likely responsible for this newborn's symptoms?