A 1-Day-Old Boy Is Diagnosed with Hyperphenylalaninemia by Newborn Screening

A 1-day-old boy is diagnosed with hyperphenylalaninemia by newborn screening.  He is placed on a special phenylalanine-restricted diet with tyrosine supplementation.  The parents are extensively counseled on the boy's condition and informed of the necessary dietary restrictions.  They are also instructed to return to his physician for regular follow-up visits.  Several months later, laboratory test results indicate that the infant has a normal serum phenylalanine level.  Careful examination, however, reveals some neurological abnormalities, including axial hypotonia and microcephaly.  Further workup is notable for elevated prolactin, and his physician suspects a cofactor deficiency.  Which of the following enzymes is most likely deficient in this patient?

A) Dihydrobiopterin reductase
B) Dopamine β-hydroxylase
C) Phenylalanine hydroxylase
D) Phenylethanolamine N-methyltransferase
E) Tyrosinase

Correct Answer:

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