Garrod's information about the enzyme alteration resulting in alkaptonuria led to further elucidation of the same pathway in humans. Phenylketonuria (PKU) occurs when another enzyme in the pathway is altered or missing, resulting in a failure of phenylalanine (phe) to be metabolized to another amino acid: tyrosine. Tyrosine is an earlier substrate in the pathway altered in alkaptonuria. How might PKU affect the presence or absence of alkaptonuria?
A) It would have no effect, because PKU occurs several steps away in the pathway.
B) It would have no effect, because tyrosine is also available from the diet.
C) Anyone with PKU must also have alkaptonuria.
D) Anyone with PKU is born with a predisposition to later alkaptonuria.
E) Anyone with PKU has mild symptoms of alkaptonuria.
Answer: B
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