Why is phenylketonuria resulting from dihydropteridine reductase deficiency a more serious disorder than PKU resulting from phenylalanine hydroxylase deficiency?

pteridine reductase deficiency alters all reactions known to be dependent on tetrahydrobiopterin (BH4)  

Explanation:

Hyperphenylalaninemia is a condition characterized by elevated concentrations of the amino acid phenylalanine in the blood. Dihydropteridine reductase deficiency (DHPR) is a severe type of hyperphenylalaninemia caused by the deficiency of tetrahydrobiopterin (BH4). BH4 helps to convert amino acids, including phenylalanine, to precursors of neurotransmitters. BH4 is an essential non-protein co-factor of enzymes associated with the biosynthesis of dopamine, serotonin, noradrenaline and adrenaline. DHPR is an inherited recessive disorder whose symptoms include mental deficiency, poor swallowing (dysphagia) and an abnormally small skull (microcephaly).

i believe it would be c: tons.