An inherited disorder in which the enzyme that converts the amino acid phenylalanine into tyrosine (another amino acid) is defective. Unless phenylalanine is excluded from the diet, it builds up in the body and causes severe learning difficulties. All newborn babies are given the Guthrie test for phenylketonuria. Affected babies show few signs of abnormality, but, unless phenylalanine is avoided, they develop neurological disturbances including epilepsy. They may have blonde hair and blue eyes, and their urine may have a mousy odour. Many have eczema. Phenylalanine is found in most proteincontaining foods and in artificial sweeteners. A specially modified diet is generally recommended throughout life, (and especially during pregnancy, because high phenylalanine levels in the mother can damage the fetus).


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