Children affected by this condition are normal at birth and in the neonatal period.
Developmental delay becomes apparent at about 3 months when the child fails to sit, walk, or talk at the appropriate times.
The IQ of untreated patients is usually under 50. Behavioural abnormalities and microencephaly is common, and seizures occur in about 25% of patients.
Other possible clinical features of the disease include light pigmentation, blue eyes, fair hair, eczema and mousy odour, unless treated by low phenylalanine diet.
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