You would sue the person(s) or institution(s) you believed left the sponge in the wound; if you don't know which of more than one possibilities, it is, you could sue everyone who reasonably may have left it and allow them to "fight it out" about liability (who is responsible). But it would only be worth doing this if you incurred several thousand dollars in out-of-pocket (not paid by insurance) medical costs and/or suffered some lasting impairment or injury from it. Otherwise, since medical malpractice suits can be expensive (you have to hire a medical expert; they do not work cheap), you could spend as much or more on the lawsuit than you'd get back.