Snapshot A 2-month-old infant was born with blond hair, blue eyes, and fair skin. It is noticed he has an eczematous rash and is vomiting. In addition he has a musty, mousy odor. Introduction Screened for at birth in the US by one of the following Guthrie bacterial inhibition study automated fluorimetric assay tandem mass spectrometry Caused from a decrease in phenylalanine hydroxylase or tetrahydrobiopterin cofactor normally converts phenyalanine into tyrosine therefore tyrosine becomes essential phenylalanine builds up leading to excess phenylketones which cause brain damage Presentation Infants are normal at birth After a few months mental retardation is evident Present with blond, blue eyed, with fair skin mental retardation eczema musty body odor of phenyacetic acid vomiting is common and is often mistaken for pyloric stenosis Physical exam shows hyperactive deep tendon reflexes Treatment Decreased phenylalanine diet early institution is essential to prevent the development of brain damage Increase tyrosine in diet Pregnant mothers with PKU must go back on a low phenylalanine diet before conception and through pregnancy to reduce the risk of birth defects