Well… I’m assuming you know that people with PKU are lacking the enzyme we need to break down phenylalanine and convert it to tyrosine. So when our bodies contain too much phenylalanine, there’s no way for us to get rid of it. It’s not really known why excess phenylalanine is so bad for us but it is. Somehow (don’t know how cuz I haven’t studied it that much yet) the excess phenylalanine prevents our bodies from producing myelin, which acts kind of like an insulator to the neurons in our brain, specifically the white matter. The myelin is what makes the white matter white! Without myelin, signals in our brains don’t travel as fast…kind of like electricity travelling fast through a wire (myelin) but much slower through rubber (no myelin). Hence, when our levels of phenylalanine are high, everything is slower and our brain just doesn’t function at top notch. Hopefully this was helpful!