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What is Phenylketonuria is due to deficiency of: and explain in detail?
Ref. Textbook of Biochemistry for Medical Students. Page.300
 
Phenylketonuria 

Defect in Phenyl alanine Hydroxylase
A variant, nonclassic PKU, is a result of a defective enzyme in tetrahydrobiopterin synthesis.
Mental retardation
Musty odor 
Microcephaly
Diet low in phe nyl-alanine
Avoid aspartame 
Diet important during pregnancy to prevent complication in the kid