In a newborn of a mother with phenylketonuria (PKU), increased maternal phenylalanine concentrations are associated with which finding in the newborn?

• A. Ambiguous genitalia
• B. Hydrops fetalis
• C. Macrocephaly
• D. Microcephaly

Answer: (D)

Elevated maternal phenylalanine during pregnancy is teratogenic to the fetal brain. Phenylalanine crosses the placenta, and when levels are high, it disrupts fetal brain development and growth, leading to reduced head circumference. In untreated maternal PKU, the newborn commonly shows microcephaly along with growth retardation and facial features. Ambiguous genitalia, hydrops fetalis, and macrocephaly are not typical findings of maternal PKU, making microcephaly the best description of the effect on the newborn.