Clostridium difficile is a gram-positive, anaerobic, spore-forming bacillus that produces a strong toxin. C difficile is a common cause of antibiotic-associated diarrhea and pseudomembranous colitis.

Microbiology

A recto-sigmoidoscopy revealed pseudomembranes in a patient with severe diarrhea following prolonged treatment with ampicillin. Which of the following organisms is MOST likely to be isolated?