Phenylketonuria (PKU) By Matt Demczko, MD, Mitochondrial Medicine, Children's Hospital of Philadelphia Reviewed / Revised Oct 2021 | Modified Sep 2022 View Patient Education Pathophysiology Symptoms and Signs Diagnosis Prognosis Treatment Key Points More Information Phenylketonuria is a disorder of amino acid metabolism that causes a clinical syndrome of intellectual disability with cognitive and behavioral abnormalities caused by elevated serum phenylalanine.