rare autosomal recessive disorder of the urea cycle; caused by a deficiency of the hepatic enzyme type I arginase; arginine is elevated in the blood and cerebrospinal fluid, and periodic hyperammonemia may occur; disease onset is usually in infancy or early childhood; clinical manifestations include seizures, microcephaly, progressive mental impairment, hypotonia, ataxia, spastic diplegia, and quadriparesis.