NOS2 and acute kidney injury: According to Piton et al.32, two factors may increase the plasma Cit concentration in critically ill patients: the iNOS, which increases the synthesis of extra-intestinal Cit from Arg, and acute renal failure, a frequent condition in critically ill patients which may cause a decrease in Arg synthesis from Cit in the kidneys leading to an abnormally high plasma Cit concentration32.