In Phenylketonuria the main aim of first line therapy is:

Ans is 'c' i.e. Limiting the substrate for deficient enzyme Classical (typeI) phenylketonuria is caused by deficiency of phenylalanine hydroxylase enzyme. The reaction catalyzed by this enzyme is:Deficiency of phenylalanine hydroxylase results in accumulation of phenylalanine and its byproducts (i.e. Phenyl lactate, phenylacetate and phenylpyruvate). Elevated levels of phenylalanine and its byproducts would lead to severe mental retardation if not treated in infancy.Treatment consists of a special diet low in phenylalanine.(diet must also be supplemented with tyrosine; since tyrosine becomes an essential amino acid due to deficiency of phenylalanine hydroxylase) Initiation of dietary treatment of classic phenylketonuria must occur before the child is 3 weeks of age.