Serious congenital metabolic abnormality that occurs in 1 in 15,000 births, caused by lack of the liver enzyme that converts the amino acid phenylalanine (which occurs in all protein) and which thus accumulates in excessive quantities in the blood. The brain is affected, and the child becomes backward; normal nutrition is thus poisonous. Such children seem normal at birth, but after about 6 weeks the urine smells musty, and after 4 to 6 months the mother notices signs of backwardness. The baby becomes irritable, does not want to be cuddled, vomits frequently and may have convulsions. Symptoms worsen with age, and finally the child becomes mentally deficient. Diagnosis is by blood and urine tests. New-born children are tested for the condition, and if found in time the consequences can be limited or prevented by a special low phenylalanine diet.