All the following are characteristics of phenyiketonuria EXCEPT: untreated patients suffer severe mental retardation. excretion of phenylpyruvate. treated by putting patient on a diet low in phenylalanine. deficiency or defect in phenylalanine hydroxylase. air oxidation causes urine to turn dark on standing. 7.1 point All the following are characteristics of alkoptonuria EXCEPT: homogentisate accumulation in joints may cause arthritis excretion of homogentisate air oxidation causes urine to turn dark on standing results in mental retardation due to accumulation of Phe metabolites all are true .