A pregnant woman has untreated classic phenylketonuria. Her fetus is heterozygous and has normal phenylalanine hydroxylase activity. What is the expected consequence for the infant?

• A No abnormality because maternal enzyme protects the fetus
• B Congenital microcephaly, cardiac defects, and intellectual disability due to transplacental phenylalanine excess ✓
• C Isolated transient neonatal hyperphenylalaninemia resolving by 6 months
• D Neonatal hyperammonemic coma requiring hemodialysis

Explanation

Maternal PKU syndrome arises because the placenta actively transports phenylalanine to the fetus, whose heterozygous PAH cannot clear the excess. Embryotoxic levels produce microcephaly, congenital heart disease, low birth weight, and intellectual disability. The fetus being enzymatically normal does not help since the damage reflects substrate load from the mother. Prevention requires strict maternal phenylalanine restriction before conception.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.