A 3-day-old baby refuses feeds, vomits, and becomes lethargic. This is a urea cycle defect until proven otherwise — protein breakdown releases ammonia, and the broken cycle can't convert it to urea; ammonia poisons the newborn brain (lethargy → coma). Blood ammonia is sky-high. Emergency: stop protein, give glucose (stops catabolism), dialyze, and give nitrogen-scavenger drugs. Newborn screening catches some — the urea cycle is the ammonia detox plant, and its failure is a neonatal emergency.