This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous (Bio-Chemistry), Subject name: Biochemistry
A child with hypopigmented hair and mentally retarded with mousy odour urine. Enzyme deficient is? 
Select and explain. 
A. Cystathione synthase, B. Tyrosine hydroxylase, C. Phenylalanine hydroxylase, D. Tyrosine transaminase
C: Phenylalanine hydroxylase
Answer: C: Phenylalanine hydroxylase 
Explanation: (C) Phenyalanine hydroxylase # PHENYLKETONURIA: phenylalanine hydroxylase deficiency.> Disease presents clinically with seizures, hypo pigmentation, musty odour to baby sweat and urine(due to phenylacetate, a carboxylic acid produced by oxidation of phenylketone).> Phenylalanine is first converted to tyrosine.> Hyper phenylalaninemias arise from defects in phenylalanine hydroxylase,(type I, classic phenylketonuria (PKU), frequency 1 in 10,000 births), in dihydrobiopterin reductase (types II and III), or in dihydrobiopterin biosynthesis (types IV and V).> Alternative catabolites are excreted.> Diet low in phenylalanine can prevent the mental retardation of PKU.> DNA probes facilitate prenatal diagnosis of defects in phenylalanine hydroxylase or dihydrobiopterin reductase.> Elevated blood phenylalanine may not be detectable until 3 to 4 days postpartum.> False-positives in premature infants may reflect delayed maturation of enzymes of phenylalanine catabolism.> An older and less reliable screening test employs FeCI3 to detect urinary phenylpyruvate.> FeCI3 screening for PKU of the urine of newborn infants is compulsory in many countries, but in the United States has been largely supplanted by tandem mass spectrometry.