This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Bacteriology, Subject name: Microbiology
[{'content': 'Most common organism responsible for pseudomembranous colitis is \nSelect and explain. \nA. Clostridium difficile, B. Clostridium botulism, C. Clostridium histolyticum, D. Clostridium butyricum', 'role': 'user'}, {'content': "Answer: A \nExplanation: Clostridium difficile: long, slender, gram-positive bacilli. Non-hemolytic, saccharolytic and weakly proteolytic. Causes: bloody diarrhea, pseudomembranous colitis, a common cause of nosocomial diarrhea. Pathogenesis: high molecular weight exotoxin A and B Toxin A is a potent enterotoxin, Toxin B is a cytotoxin. Reference: Textbook of Microbiology; Anathanarayan and paniker's; 10th edition; Page no: 270", 'role': 'assistant'}]