An inherited urea cycle disorder in which deficiency of the enzyme argininosuccinate lyase blocks the normal formation of urea, causing argininosuccinic acid and ammonia to accumulate. The excess ammonia is toxic to the brain, producing vomiting, lethargy, seizures, and developmental impairment, often beginning in the newborn period with catastrophic illness. Long-term management combines strict dietary protein restriction with essential amino acid supplements and nitrogen-scavenging drugs that carry waste nitrogen out through alternative pathways. Unique among urea cycle disorders, it also causes distinctive brittle, tufted hair, a visible clue to the underlying biochemistry. With early detection and meticulous dietary control, many patients achieve good outcomes, making it another success story for newborn screening and metabolic nutrition.