The term ""vitamin K"" refers to a group of chemically similar fat-soluble compounds called naphthoquinones: vitamin K1 (phytonadione) is found in plants and is the primary source of vitamin K for humans through dietary consumption, vitamin K2 compounds (menaquinones) are made by bacteria in the human gut, and vitamin K3 (menadione) is a water-soluble preparation available for adults only. Vitamin K is necessary for the liver to produce the coagulation factors II, VII, IX, and X, as well as the clotting factors protein C, protein S, and protein Z; vitamin K deficiency can result in deficiencies of these coagulation factors and excess bleeding. An injection of vitamin K is routinely given to newborn infants to prevent vitamin K deficiency bleeding, also known as hemorrhagic disease of the newborn. Vitamin K deficiency is rare in adults but may result from chronic malnutrition or an inability to absorb dietary vitamins. CRCH Definition: Fat soluble vitamin that is a coenzyme during the synthesis of the biologically active form of a number of proteins involved in blood coagulation and bone metabolism. Vitamin K's structure typically consists of a methylated naphthoquinone ring from which one or more unsaturated aliphatic side chains are attached at the 3-position of the ring. NCI-GLOSS Definition: A nutrient that the body needs in small amounts to function and stay healthy. Vitamin K helps to form blood clots (a mass that forms when blood platelets, proteins, and cells stick together) and maintain strong bones. It is fat-soluble (can dissolve in fats and oils) and is found in green leafy vegetables, broccoli, liver, and vegetable oils. Vitamin K is also made by bacteria that live in the large intestine. Not enough vitamin K can lead to bleeding and bruising."