A two-year-old child was taken to the hospital. His mother said that he vomited
frequently, especially after feedings. The child’s weight and physical development
were below normal. His hair, although dark, contained patches of white. A urine
sample treated with ferric chloride (FeCl3) gave a green color characteristic of the
presence of phenylpyruvate. Quantitative analysis of urine samples gave the results
shown in the table.
Substrate Concentration (mM)
Patients urine Normal urine

Phenylalanine 7 0.01
Phenylpyruvate 4.8 0
Phenyllactate 10.3 0
(a) Suggest which enzyme might be deficient in this child. Propose a treatment.
(b) Why does phenylalanine appear in the urine in large amounts?
(c) What is the source of phenylpyruvate and phenyllactate? Why does this pathway
(normally not functional) come into play when the concentration of phenylalanine
rises?
(d) Why does the boy’s hair contain patches of white?