A 3-day-old neonate has a musty (mousy) odor, fair skin and hair compared to siblings, and later develops intellectual disability. Urine ferric chloride test gives a green color. Newborn screening would have detected this condition by measuring which amino acid?
- A Phenylalanine ✓
- B Tyrosine
- C Homocysteine
- D Leucine
Correct answer: A. Phenylalanine
Explanation
This is Phenylketonuria (PKU), caused by deficiency of phenylalanine hydroxylase, leading to accumulation of phenylalanine (which is converted to phenylpyruvate and phenylacetate — giving the mousy odor). The ferric chloride test detects phenylpyruvate in urine (green color). Fair pigmentation results from inhibition of tyrosinase by excess phenylalanine, reducing melanin synthesis. Newborn screening measures elevated phenylalanine in blood (Guthrie test / tandem mass spectrometry). Early dietary restriction of phenylalanine prevents intellectual disability.
Reference: Ghai Essential Pediatrics, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.