In Reply.—For most metabolic disorders, a diagnosis based on a single symptom would not be acceptable. Hyperphenylalaninemia is a symptom that in a newborn infant suggests either complete or partial PH deficiency (PKU), dihydropteridine reductase deficiency, or deficiency in biopterin synthesis. The last two defects were recognized only after liver biopsy specimens showed near-normal PH activity in patients who were thought to have PKU.
Phenylketonuria has been studied intensively for nearly 30 years. There are as many unanswered questions today as there were then. We have no clue as to why some untreated patients with classic PKU attained normal mental development, while the majority were severely mentally retarded.1 There is still no consensus as to which children with hyperphenylalaninemia need a phenylalanine-restricted diet to ensure normal intellectual development.2,3 There is no agreement as to whether the phenylalanine-restricted diet can be terminated without adverse effects.4 Many other