Newborn screening flags high phenylalanine; the baby is started on special formula. This is PKU (phenylketonuria) — phenylalanine hydroxylase is missing, so phenylalanine can't become tyrosine; it accumulates and its byproducts poison the developing brain (severe intellectual disability, musty odor). Caught by the heel-prick test on day 2–3, a strict low-phenylalanine diet from birth prevents damage — the child grows up normal. It is the triumph of newborn screening: a devastating disease neutralized by diet, but only if caught early.