a fat-soluble vitamin, synthesized constantly by intestinal bacteria in mammals and occurring in certain green vegetables, fish meal, etc., that promotes blood clotting and is required for the synthesis of prothrombin by the liver: the two naturally occurring varieties are vitamin K1, C31H46O2, found chiefly in alfalfa leaves, and vitamin K2, C41H56O2, found chiefly in fish meal: vitamin K3 ( menadione) and vitamin K4 are prepared synthetically