Lack of weight gain isn’t a symptom of untreated PKU from what I know. Here are some sypmtoms of untreated PKU: vomitting headaches tremors/shakes lack of energy lack of concentration irritability siezures and eventually mental retardation
If she does end of having PKU and is treated (low protein diet, metabolic formula and regular blood tests) then she will live a happy and normal life. The only symptom that people with PKU have if they are treated is that they usually have a lighter complextion then their siblings. Which usually, but not always, means blue eyes, blonde hair and very light skin.
If you and her parents think she has PKU I would suggest getting the Newborn screening test redone on her. The first time it would have been done (shortly after birth) they might have done it to early to let Phenylalanine accumulate in the blood. I hope everything gets figured out! Breanna 16 CPKU