George, a 55-year-old retired businessman with a diagnosis of myelofibrosis, underwent an allogeneic stem cell transplantation from his human leukocyte antigen-matched brother in June 2006. He was admitted to the hospital for a possible flare of graft-versus-host disease (GVHD) of the gut. His medications included tacrolimus, budesonide, and bechlamethasone for immunosuppression and pantoprazole. A stool sample was positive for Clostridium difficile toxin A on October 31, 2006, and he was started on oral metronidazole.