Until recently, Menadione Sodium Bisulfite Complex was only one of many obscure ingredients listed on some cat food labels. You may also have seen it as "Vitamin K3" among other names. The use of Menadione, as I'll refer to it here, has become increasingly controversial as a cat food ingredient. One of Menadione's biggest opponents is Susan Thixon, founder of PetSumer Report Online. She writes for Big Cat Rescue that Menadione has carcinogenic effects and "is toxic to kidneys, lungs, liver, mucous membranes." The FDA takes a less cautious approach and has no present restrictions on the use of Menadione in pet foods, if you can read through its obscure language in this PDF file.
I've noticed Menadione listed in the ingredients of some of the premium foods on my current lists. Since the jury is still out on Menadione, I will not remove those foods from the lists, but will note the inclusion of Menadione in their descriptions, and in other foods, going forward. Read more on Menadione.