Which of the following organism is known to cause pseudomembranous colitis?

Pseudomembranous colitis is caused by Clostridium difficile a gram positive bacillus. Colitis is thought to result from overgrowth of this organism after depletion of the normal commensal flora of the gut with the use of antibiotics. Pathogenic changes associated with C difficile result from production of toxin A and B. It can be diagnosed by the presence of these toxins in the stool. It can also be diagnosed endoscopically by detecting the presence of characteristic ulcers, plaques and pseudomembranes. Patients with mild disease are treated with metronidazole. Oral vancomycin is used as second line agent in patients allergic to metronidazole or in patients with recurrent disease. Ref: Bullard Dunn K.M., Rothenberger D.A. (2010). Chapter 29. Colon, Rectum, and Anus. In T.R. Billiar, D.L. Dunn (Eds), Schwaz's Principles of Surgery, 9e.