Newborn babies with PKU do not show any symptoms. At about 3 to 6 months, babies with untreated PKU will begin to be less active. They show little interest in everything around them. By 1 year, the child may have severe developmental delays.
Children with PKU have lower levels of melanin, the substance that gives color to hair and skin. That's because when phenylalanine is broken down, one of its products is used to make melanin. As a result, children with PKU often will have pale skin, blond hair and blue eyes. Dry skin; eczema; and a "musty" odor resulting from the buildup of phenylalanine in hair, skin and urine are also common. Other signs and symptoms may include irritability, muscle stiffness, seizures, a small head and short stature.