Question

Assume you were a physician and a mother with two mentally retarded children consulted you. In the course of an interview, you learned that the urine of the children had a curious odor. You later tested their urine with ferric chloride and found that, whereas normal urine gives a brownish color, the children’s urine stained green. You deduced that the chemical responsible must be phenylpyruvic acid. Because of chemical similarity to phenylalanine, it seemed likely that this substance had been formed from phenylalanine in the blood, but there was no assay for phenylalanine. However, a certain bacterium could convert phenylalanine into phenylpyruvic acid; therefore, the level of phenylalanine could be measured by using the ferric chloride test. The children were indeed found to have high levels of phenylalanine in their blood, and the phenylalanine was probably the source of the phenylpyruvic acid. This disease, which came to be known as phenylketonuria (PKU), was shown to be inherited and caused by a recessive allele. It became clear that phenylalanine was the culprit and that this chemical accumulated in PKU patients and was converted into high levels of phenylpyruvic acid, which then interfered with the normal development of nervous tissue. This finding led to the formulation of a special diet low in phenylalanine, which could be fed to newborn babies diagnosed with PKU and which allowed normal development to continue without retardation. Indeed, it was found that after the child’s nervous system had developed, the patient could be taken off the special diet. However, tragically, many PKU women who had developed normally with the special diet were found to have babies who were born mentally retarded, and the special diet had no effect on these children.

a. Why do you think the babies of the PKU mothers were born retarded?

b. Why did the special diet have no effect on them?

c. Explain the reason for the difference in the results between the PKU babies and the babies of PKU mothers.

d. Propose a treatment that might allow PKU mothers to have unaffected children

Answer 1