Complex sentence: Phenylketonuria (PKU) is an inborn error of amino acid metabolism (the most common in the UK) caused by absent or virtually absent phenylalanine hydroxylase (PAH) enzyme activity.
Split into simpler sentences: Phenylketonuria (PKU) is an inborn error of metabolism involving impaired metabolism of phenylalanine, one of the amino acids. Phenylketonuria is caused by absent or virtually absent phenylalanine hydroxylase (PAH) enzyme activity.