Which of the following is the causative organism of Pseudomembranous colitis?

Pseudomembranous colitis is caused by Clostridium difficile. It is the most common cause of diarrhea that develops in association with the use of antimicrobial agents (ampicillin, cephalosporin and clindamycin). This condition is associated with intense inflammation and the formation of a pseudomembrane composed of inflammatory debris on the mucosal surface. Diagnosis is made by direct detection of toxins in the stool. Enzyme immunoassays, which demonstrate toxin A and/or B in stool are widely used. Ref: Sherris Medical Microbiology, 5e, Chapter 29