The enzyme deficient in Phenyl Ketonuria is:

Ref: Harpers illustrated Biochemistry, 28th editionExplanation: (See figure below)Tetrahydro biopterin (H4B) dependent reaction.In case of deficiency of phenyl alanine hydroxylase, phenyl alanine is converted in to phenylpyruvate & phenylacetate. accumulation of which results in phenyl ketonuria. FeaturesMental retardationLight color skin & hairEczema & seizures| Plasma phenylalanine| Phenylacetate & phenyl pyruvate in urineTreatment consists of limiting phenylalanine intake to levels barely adequate to support growth. Tyrosine, an essential nutrient for individuals with phenylketonuria, must be supplied in the diet.