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**Phenylketonuria (PKU)** in humans is caused by:
AA dominant autosomal allele preventing tyrosine production
BA recessive autosomal allele leading to absence of phenylalanine hydroxylase
CA sex-linked recessive allele on the X chromosome
DA Y-linked allele inherited father-to-son
Answer & Solution
Correct answer: B. A recessive autosomal allele leading to absence of phenylalanine hydroxylase
PKU is an autosomal recessive metabolic disorder. Homozygotes lack the enzyme phenylalanine hydroxylase, which normally converts phenylalanine → tyrosine. Phenylalanine and derivatives accumulate in blood/CSF, causing brain damage and mental retardation; excess is excreted in urine (hence 'phenyl-ketone-uria').
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