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 K, CHO, found chiefly in alfalfa leaves, and vitamin K, CHO, found chiefly in fish meal: vitamin K ( menadione) and vitamin K are prepared synthetically