[Atypical form of phenylketonuria caused by lack of dihydrobiopterin synthetase activity].

A case of non-typical phenylketonuria produced by the inactivity of dihydrobiopterin synthetases is presented. Dihydrobiopterin synthetases are enzymes converting neopterin to biopterin. Presented case indicates a possibility of erroneous classification of hyperphenylalaninemia due to BH4 deficit, if the complete differential diagnosis is not performed. A complete differential diagnosis is necessary in all cases of hyperphenylalaninemia distinguished in neonates during screening tests.