<prompt> In Phenylketonuria the main aim of first-line therapy isReplacement of the defective enzymesReplacement of the deficient productLimiting the substrate for deficient enzymeGiving the missing amino acid by diet  <response>: In PKU Phenylalanine hydroxylase enzyme is deficient.So alternative pathways are opened. Leading to the formation of Phenylketone,Phenyllactate and Phenylacetate. This can be prevented by limiting the substrate for deficient enzyme.Ref: DM Vasudevan - Textbook of Biochemistry, 7th edition, page no: 236 