A lipid cofactor that is required for normal blood clotting. Several forms of vitamin K have been identified: vitamin k 1 (phytomenadione) derived from plants, vitamin k 2 (menaquinone) from bacteria, and synthetic naphthoquinone provitamins, vitamin k 3 (menadione). vitamin k 3 provitamins, after being alkylated in vivo, exhibit the antifibrinolytic activity of vitamin K. Green leafy vegetables, liver, cheese, butter, and egg yolk are good sources of vitamin K.