A newborn screening picks up elevated phenylalanine (2400 µmol/L) in a formula-fed infant at day 5. Confirmatory testing confirms classical phenylketonuria (PAH deficiency). The MOST critical window for starting dietary phenylalanine restriction to prevent intellectual disability is:

• A Before 1 month of age (ideally within 7-10 days of birth) ✓
• B Before 3 months of age
• C Within 6 months is sufficient as brain development continues
• D Immediately but only if IQ testing shows decline

Explanation

PKU treatment must begin within the first 7-10 days of life — ideally before the end of the first month — to prevent irreversible phenylalanine-induced neurotoxicity and intellectual disability. Untreated PKU causes severe intellectual disability (IQ often <30), microcephaly, behavioral problems, and seizures through phenylalanine accumulation impairing myelin synthesis and neurotransmitter production. Newborn screening programs target day 2-5 specifically to allow treatment within this critical window. Delaying to 3 months results in measurable IQ loss. The dietary restriction of phenylalanine (low-phe formula supplemented with tyrosine) must be lifelong, particularly critical during childhood and pregnancy.

Reference: Ghai Essential Pediatrics, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.