What is fenilketonuria, that most lighting beverages next to aspartame suggets to study?

What is fenilketonuria, that most lighting beverages next to aspartame suggets to study?
Phenylketonuria or PKU is a disease that stops the liver breaking down phenylalanine to tyrosine so phenylalanine build up in the blood and eventually go to the brain which causes brain weaken. Everyone is tested at birth and suffers must avoid phenylalanine such as in diet fizzy drinks
its phenyl at the commencing, not fenil.
It has something to do beside brain chemistry. I dont think they hold a certain chemical within their body to break down phenylalanine, and so if they eat aspartame it can make happen mental retardation and even death. Bad stuff.