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A newborn male infant was diagnosed as having phenylketonuria (PKU), and immediately placed on diet low in phenylalanine ; careful compliance with the diet and frequent monitoring of the patient’s plasma Phenyl alanine level resulted in the level being maintained at the lower limit of the normal range. The patient appeared to be developing normally until 4 months of age, when he developed truncal hypotonia and spasticity of the limbs. Despite being on a low-phenylalanine diet, at 5 months the patient had several grand mal (epileptic) seizures. After an abnormal Phenyl alanine-loading test, the patient’s urine was found to have a markedly elevated urinary biopterin concentration.

Which of the following enzymes is most likely deficient in this patient?

A. Dihydropterin reductase

B. GTP cyclohydrolase I

C. Phenylalanine hydroxylase

D. Tryptophan hydroxylase

E. Tyrosine hydroxylase

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