A 3-Year-Old Boy Who Recently Immigrated to the United States

A 3-year-old boy who recently immigrated to the United States is brought to the physician by his parents because he has not yet begun to walk or speak.  Assessment of his developmental milestones shows severe intellectual disability.  He dies 6 months later from refractory seizures resulting in respiratory failure.  Autopsy shows pallor of the substantia nigra, locus ceruleus, and vagal nucleus dorsalis.  The underlying condition responsible for this patient's death is most likely caused by a deficiency of which of the following enzymes?

A) Branched-chain ketoacid dehydrogenase
B) Dopamine hydroxylase
C) Homogentisic acid oxidase
D) Phenylalanine hydroxylase
E) Tyrosinase

Correct Answer:

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