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Question 25

A 23 yo with classic phenylketonuria (PKU) presents for pre-conception consultation. To reduce the risks of fetal intellectual disability and birth defects, what is the most important recommendation for her pre-pregnancy diet?

Answer Choices:

Low in fat

High in carbohydrates

Low in protein

High in folate

Correct Answer:

Low in protein


  • PKU is an autosomal recessive disorder of Phe metabolism (deficient phenylalanine hydroxylase)

  • Increased blood Phe levels are toxic to the developing brain

  • Mainstay of treatment: dietary restriction of Phe

  • Fetal brain and heart are vulnerable to the high concentrations of Phe

  • Levels of Phe in fetal blood are higher than maternal blood because Phe crosses the placenta by active transport

  • If not on a restricted diet, children born to women with PKU have a 92% risk of developmental delay, 73% risk of microcephaly and 12% risk of congenital heart disease

  • Fetal risks are lowered by maintaining Phe levels 2-6 mg/dL before pregnancy or by 8 weeks gestation

  • A Phe-free diet should involve avoidance of foods rich in protein, those containing aspartame, nonwheat flour, soya or beer.