The Collaborative International Dictionary of English v.0.48:

vitamin K3 \vitamin K3\ n.
   menadione, (C11H8O2) being chemically
   2-methyl-1,4,-naphthoquinone. Having no side chain in the 3
   position, it cannot exert all functions of a true K vitamin
   and the designation as vitamin K3 has been discouraged
   --[MI11]

   Syn: vitaminK2(0); menaphthone.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

vitamin K3
    n 1: a form of vitamin K [syn: vitamin K3, menadione]