Why don't PKU patients necessarily have alkaptonuria?

Phenylketonuria (PKU) and alkaptonuria are distinct genetic disorders caused by mutations in different enzymes. PKU results from a deficiency in the enzyme phenylalanine hydroxylase, leading to the accumulation of phenylalanine. In contrast, alkaptonuria is caused by a deficiency in the enzyme homogentisate oxidase, resulting in the accumulation of homogentisic acid. Therefore, PKU patients do not necessarily have alkaptonuria because the underlying metabolic pathways and enzymatic deficiencies are unrelated.