Genetic and Pediatric Diseases · USMLE Step 1

Which metabolic pediatric disease results from inability to degrade an amino acid, leading to intellectual disability unless detected by newborn screening and treated early with dietary restriction?

  1. Cystic fibrosis
  2. Phenylketonuria (PKU)
  3. Marfan syndrome
  4. Turner syndrome
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Correct answer: Phenylketonuria (PKU)

Phenylketonuria (PKU) is an inborn error of metabolism due to phenylalanine hydroxylase deficiency; if untreated, phenylalanine accumulates causing intellectual disability — early detection and dietary management prevent this.

Difficulty: Medium Question 14 of 20

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