A 3-day-old infant presents with vomiting, poor feeding, and a 'mousy' odor. Physical examination reveals fair skin and blond hair. Laboratory tests show elevated phenylalanine levels. Which enzyme is most likely deficient in this patient?

✓ Correct Answer: Option B

Elevated phenylalanine and the characteristic 'mousy' odor indicate Phenylketonuria (PKU), caused by a deficiency of phenylalanine hydroxylase. This enzyme converts phenylalanine to tyrosine, and its deficiency leads to toxic accumulation of phenylalanine.