PHENYLKETONURIA is a hereditary metabolic disorder characterized by an elevation of plasma phenylalanine and the excretion of phenylpyruvic acid in the urine. In the past, it was believed that nearly all patients afflicted with this disorder were mentally retarded. In a survey covering all published cases, Jervis1 found that only three out of 330 phenylketonuric children (0.9%) had an intelligence quotient above 70. In 1960, Knox2 reported that there were 20 known untreated patients with phenylketonuria having "high grade" intelligence. Of these, eight patients had IQs between 60 and 69; six patients had IQs between 70 and 79; three patients had IQs between 80 and 89; and three patients had IQs over 90. More recently, Knox3 has expressed the view that no single factor can account for the relatively mild intellectual defect in these "atypical" phenylketonuric children. He believes that some of these have a biochemical defect
Hsia DY, O'Flynn ME, Berman JL. Atypical Phenylketonuria With Borderline or Normal Intelligence. Am J Dis Child. 1968;116(2):143–157. doi:10.1001/archpedi.1968.02100020145005
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