A 4-year-old child presents with developmental delay, a distinctive 'mousy' odor, and fair skin and hair. These findings are most consistent with a deficiency in which enzyme?

The correct answer to the question "A 4-year-old child presents with developmental delay, a distinctive 'mousy' odor, and fair skin and hair. These findings are most consistent with a deficiency in which enzyme?" is Option B: Phenylalanine hydroxylase.

The other options — A) Homogentisate oxidase, C) Tyrosinase, D) Galactose-1-phosphate uridyltransferase — are incorrect for this USMLE Exam Prep question. As explained above: The symptoms describe Phenylketonuria (PKU), caused by a deficiency in phenylalanine hydroxylase. This enzyme converts phenylalanine to tyrosine. Accumulation of phenylalanine leads to neurotoxicity …