yes it is possible for untreated PKU t have autistic symptoms. Sometimes children with autism have a musky smell about them and often have extra protein in their urine. This also happens in untreated PKU cases. Also in some cases people with untreated PKU tend to be extremely introverted and antisocial, also have outbursts sometimes. These are also symptoms of Autism. This isn’t just my opinion, I found this out from my Abnormal Psychology book (mental disorders). So the answer is yes untreated PKU and autism have similar symptoms.