A 20-year old female who is 2 months pregnant remembers that she had phenylketonuria (PKU) as a child and required a special diet. Tests confirm markedly elevated maternal serum levels of phenylalanine and phenylacetic acid. Genetic studies have not been performed on the father. What should the physician tell the parents regarding the welfare of the child?

A. Childhood phenylalanine restriction is sufficient to protect the health of her child.
B. Further information is required to ascertain if the fetus is at risk.
C. The fetus is at no health risk if it is heterozygous for the PKU gene.
D. The fetus is at no health risk if phenylalanine levels are normalized by the third trimester.
E. The mother's hyperphenylalaninemia may have already harmed the fetus.

I think it has been hurted.

The correct answer is E. Phenylalanine crosses the placenta and, if maternal serum levels are elevated, acts as a teratogen to the developing fetus. This condition is known as maternal PKU. Although the mother can fare well with substantial elevations in serum phenylalanine concentration, the children born to such women are usually profoundly retarded and may have multiple birth defects.
Although dietary modifications (choice A) can prevent the neurological and dermatologic manifestations of PKU in a child, the fetus is still at risk from maternal PKU.

Further information regarding the cause of this woman's hyperphenylalaninemia (choice B) is not needed, since the fetus is exposed to teratogenic levels of phenylalanine.

Children born to mothers with untreated PKU develop maternal PKU even if they are heterozygous for the PKU gene (choice C). Fetal phenylalanine hydroxylase cannot compensate for the high maternal levels of phenylalanine.

The critical period in development during which teratogenic materials affect the growing organs is between the 3rd and 8th weeks of gestation. By the end of the 2nd month (compare with choice D), the damage caused by the maternal PKU has already occurred.