<prompt> Pseudomembranous colitis is associated mostly with which drug?ErythromycinAmpicillinVancomycinCiprofloxacin  <response>: Ans. b Ampicilin. Ref. Harrisons principles of internal medicine, 18thChapter 129.Pseudomembranous colitisClostridium difficile-associated disease CDAD It is a unique colon infection that is acquired almost exclusively in association with antimicrobial use and the consequent disruption of the normal colonic flora. C. difficile is an obligately anaerobic, gram, spore-forming bacillus whose spores are found wide in nature. The agents responsible include:- Most frequently 2nd and 3rdgeneration cephalosporins cefotaxime, ceftriaxone, cefuroxime, and ceftazidime.- Clindamycin, ampicillin, and other cephalosporins- Penicillinb-lactamase-inhibitor combinations such as ticarcillin clavulanate and piperacillintazobactam pose significantly less risk.- However, all antibiotics, including vancomycin and metronidazole the agents most commonly used to treat CDAD, have been found to carry a risk of subsequent CDAD. Spores of toxigenic C. difficile are ingested, survive gastric acidity, germinate in the small bowel, and colonize the lower intestinal tract, where they elaborate two large toxins:- Toxin A, an enterotoxin , and a potent neutrophil chemoattractant- Toxin B, a cytotoxin. The diagnosis of CDAD is based on a combination of clinical criteria:- 1 diarrhea 3 unformed stools per 24 h for 2 days, with no other recognized cause; plus- 2 toxin A or B detected in the stool, toxin-producing C. difficile detected by stool culture, or pseudomembranes seen in the colon. Rx:- Metronidazole is the drug of first choice and the least expensive agent.- Vancomycin as first-line treatment is discouraged as it may increase the incidence of vancomycin-resistant enterococci in hospitals. 