Ans. B: Clostedium difficile Pseudomembranous colitis is an acute inflammatory disease of the colon that in mild cases may appear as minimal inflammation or edema of the colonic mucosa. In more severe cases, the mucosa often is covered with loosely adherent nodular or diffuse exudates. These raised exudative plaques are 2-5 mm in size. Coalescence of these plaques generates an endoscopic appearance of yellowish pseudomembranes lining the colonic mucosa, Pseudomembranous colitis usually is associated with antibiotic use, which may alter the balance of normal gut flora and allow overgrowth of ceain organisms. C difficile, a gram-positive, spore-forming, anaerobic bacillus, is isolated in almost all of the cases of Pseudomembranous colitis. Clindamycin, lincomycin, ampicillin, and cephalosporin have been implicated in most of the repoed cases, but any antimicrobial agent (including antifungal, antiviral, and metronidazole) could incite the disease, regardless of the amount administered or the route of administration. Rare cases have been related to Staphylococcus aureus, Salmonella species, Clostridium perfringens, Yersinia species, Shigella species, Campylobacter species, cytomegalovirus, Entamoeba histolytica, and Listeria species. Conditions other than antimicrobial administration could predispose to C difficile pseudomembranous colitis. Such conditions include bowel ischemia, recent bowel surgery, uremia, dietary change, change in bowel motility, malnutrition, chemotherapy, shock, and Hirschsprung disease.