Experimental Factor Ontology entry EFO_0004618 (vitamin K measurement) is is a quantification of Vitamin K or Phylloquinone, a polycyclic aromatic ketone, based on 2-methyl-1,4-naphthoquinone, with a 3-phytyl substituent. It is a fat-soluble vitamin that is stable to air and moisture but decomposes in sunlight. It is found naturally in a wide variety of green plants, particularly leaves, since it functions as an electron acceptor during photosynthesis, forming part of the electron transport chain of Photosystem I. Also known as: phylloquinone measurement.