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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.
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yessssssssssss
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.
