Pseudomembranous colitis is associated with use of antibiotics . due to use of antibiotics normal gut flora disturbe and clostridium difficile over grow and produce enterotoxin called toxin A. it leads to diarrhea.and also it produce cytotoxic toxin B and damage the colonic mucosa .clostridium difficile is a gram positive anaerobic spore forming rods .Treatment is discontinued initial antibiotic and start oral metronidazole or vancomycin. vancomycin poorly absorbed from gut