Amino Acids, Proteins and Porphyrins
Which one of the following statements concerning a one-week-old male infant with undetected classic phenylketonuria is correct?
|A||Tyrosine is a nonessential amino acid for the infant.|
|B||High levels of phenylpyruvate appear in his urine.|
|C||Therapy must begin within the first year of life.|
|D||A diet devoid of phenylalanine should be initiated immediately.|
a. Phenyllactate, phenylacetate, and phenylpyruvate, which are not normally produced in significant amounts in the presence of functional phenylalanine hydroxylase, are elevated in PKU, and appear in the urine.
b. In patients with PKU, tyrosine cannot be synthesized from phenylalanine and, hence, becomes essential and must be supplied in the diet. Treatment must begin during the first seven to ten days of life to prevent mental retardation.
c. Discontinuance of the phenylalanine-restricted diet before eight years of age is associated with poor performance on IQ tests.
d. Adult PKU patients show deterioration of attention and speed of mental processing after discontinuation of the diet. Elevated levels of phenylalanine are teratogenic. Lifelong restriction of dietary phenylalanine is, therefore, recommended.