If an Infant Is Found to Have Hyperphenylalaninemia Based on a Newborn

If an infant is found to have hyperphenylalaninemia based on a newborn screen, but this defect doesn't respond to a low phenylalanine diet, what type of molecular defect would be suspected?

A) Null mutation for phenylalanine hydroxylase
B) Dominant negative mutation for phenylalanine hydroxylase
C) A defect in tetrahydrobiopterin (BH₄) metabolism
D) A mutation in tyrosine hydroxylase
E) A mutation in tryptophan hydroxylase