Arginase deficiency is a rare autosomal recessive amino acid metabolism disorder characterized clinically by variable degrees of hyperammonemia, developing from about 3 years of age, and leading to progressive loss of developmental milestones and spasticity in the absence of treatment. The disease is Argininemia (Orphanet_90, an Orphanet rare-disease nomenclature term). Also known as: Arginase deficiency, Hyperargininemia.