anomaly in the amount of phenylalanine (2-amino-3-phenylpropanoic acid), an aromatic amino acid that is alanine in which one of the methyl hydrogens is substituted by a phenyl group; it is an essential alpha-amino acid and the precursor of tyrosine, the monoamine neurotransmitters dopamine, norepinephrine and epinephrine, and the skin pigment melanin