the perp is a guy who has some relation to the hospital, and he poisons himself by ingesting an antidote which reverses the poison given to him and another patient which was allegedly his friend.... or something...
the only thing i remember is Mark Sloan realizes who is to blame because of a small red dot that appears on a bottle of iodine or something during the operating procedure. Mark catches him out by feigning illness requiring an operation where he (and the med team) plant a surgical bottle with a red dot on it and the killers conscience makes him confess during the mock operation. also there was the deal of the surgical gloves being sabotaged with tiny pin pricks so that the patient would absorb the contaminated substance thru their fingers....
anyhoo, wiki and the like is not helping me out this time...