What failure occurs in patients with PKU that causes health issues related to amino acid metabolism?

The correct response relates to the specific metabolic pathway involved in phenylketonuria (PKU). In patients with PKU, there is a genetic mutation that leads to a deficiency in the enzyme phenylalanine hydroxylase. This enzyme is critical for converting phenylalanine, an essential amino acid obtained from dietary proteins, into tyrosine, another important amino acid that plays a role in various physiological processes, including the production of neurotransmitters.

When the conversion of phenylalanine to tyrosine fails, it results in an accumulation of phenylalanine in the body, which can be toxic at high levels, especially to the brain. This accumulation is what leads to the neurological issues and developmental delays commonly associated with PKU. Furthermore, because tyrosine is not being synthesized from phenylalanine, other metabolic pathways that depend on tyrosine, such as the production of dopamine, norepinephrine, and epinephrine, can be adversely affected, leading to further complications.

Understanding this metabolic failure clarifies why managing phenylalanine intake through dietary restrictions is critical in PKU to prevent the harmful effects of excess phenylalanine in the system.