A 9-month-old infant is brought to the clinic for developmental assessment. The mother notes that the child has poor head control, cannot sit, has fair skin, blue eyes, musty (mousy) body odor, and recurrent eczema. What metabolic disorder is suspected?

Answer and explanation

Correct answer: D. Phenylketonuria (PKU)

  1. Step 1: Phenylketonuria (PKU) is an autosomal recessive inborn error of metabolism caused by deficiency of phenylalanine hydroxylase (PAH), impairing conversion of phenylalanine to tyrosine.
  2. Step 2: Accumulation of phenylketones causes progressive intellectual disability, microcephaly, hypopigmentation (fair hair/skin due to impaired melanin synthesis), musty body odor, and eczema.
  3. Step 3: Early newborn screening and dietary phenylalanine restriction prevent neurological damage.
Answer
Phenylketonuria (PKU).
Why not the others: 'Galactosemia' presents in neonates with cataracts, jaundice, and E. coli sepsis following milk ingestion; 'MSUD' features sweet maple-syrup-smelling urine, encephalopathy, and branched-chain ketoacidosis; 'Gaucher disease' is a lysosomal storage disease with hepatosplenomegaly and bone crises.

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