Phenylketonuria is a condition caused by a deficiency of an enzyme (phenylalanine hydroxyls) in the body. Because there is an absence of the necessary enzyme, phenylalanine cannot be converted to tyrosine, an amino acid. As a result, phenylalanine builds up in the body and causes damage to the brain.

Children with phenylketonuria usually appear normal at birth, but in the first two moths vomiting and irritability are frequent. By nine months of age, delayed development becomes apparent. The behaviour of phenylketonuria children is often autistic like. Phenylpyruvic acid is excreted in the urine - a urine test is therefore used to identify the condition. Treatment consists of a low phenylalanine diet, monitored by regular blood phenylalanine measurements.