Phenylketonuria is a genetic disease of the group of enzymopathies. It is associated with impaired metabolism of amino acids, mainly phenylalanine. Failure to follow a low protein diet causes an accumulation of phenylalanine and its toxic products. This leads to deasese of the central nervous system and to impaired mental development (phenylpyruvic oligophrenia). What kind of compound is formed in the body during oxidative deamination of phenylalanine?