This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteria, Subject name: Microbiology
Most common organism responsible for, pseudomembranous colitis is: 
Select and explain. 
A. Clostridium difficile, B. Clostridium botulism, C. Clostridium histolyticum, D. Clostridium butyricum
A: Clostridium difficile
Answer: A: Clostridium difficile 
Explanation: Ans. is 'a' i.e., Clostridium difficile(Ref: Harrison, 19th/e, p. 1091, 1092 and 17th/e, p. 819, 820; Ananthanarayan, 9th/e, p. 265 and 8th/e, p. 263)* Pseudomembranous colitis is the advanced form of Clostridium difficile-associated disease (CDAD).* Pseudomembranous colitis is diagnosed by detection of one or both C difficile toxins in stool and by endoscopic observation of pseudo membranes or micro abscesses in patients who have diarrhea and have been given antibiotics.