A newborn screening test flags an infant for elevated phenylalanine. Confirmatory plasma amino acid analysis shows phenylalanine 1200 µmol/L (normal <120). The infant is placed on a phenylalanine-restricted diet. The most critical long-term monitoring parameter aside from phenylalanine levels is:
- A Blood glucose
- B Serum calcium
- C Tyrosine levels (ensure adequate supplementation) ✓
- D Thyroid function
Explanation
In phenylketonuria (PKU), phenylalanine hydroxylase deficiency impairs conversion of phenylalanine to tyrosine, making tyrosine a conditionally essential amino acid. Phenylalanine-restricted diets also inadvertently reduce tyrosine intake. Tyrosine is necessary for neurotransmitter synthesis (dopamine, norepinephrine), thyroid hormone, and melanin production; deficiency can impair growth and cognition independently. Therefore, tyrosine supplementation and its monitoring are essential alongside phe restriction. Glucose, calcium and TSH are not the primary metabolic monitoring targets in PKU management.
Reference: Ghai Essential Pediatrics, 10th ed.
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Written and medically reviewed by the StethoPrep medical team.