A number of conditions may be associated with an increased plasma phenylalanine.

DIfferential diagnosis of hyperphenylalaninemia:

(1) decreased activity of phenylalanine hydroxylase (PAH)

(1a) classic phenylketonuria with no PAH activity

(1b) partial PAH activity

(2) disorder of BH4 metabolism

(2a) guanosine triphosphate cyclohydrolase (GTPCH) deficiency

(2b) 6-pyruvoyl tetrahydropterin synthase (PTPS) deficiency

(2c) pterin-4 acarbinolamine dehydratase (PCBD) deficiency

(2d) dihydropteridine reductase (DHPR) deficiency

(3) transient elevation of plasma phenylalanine

(3a) liver disease

(3b) prematurity

(3c) sepsis

(3d) severe burns

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