Final answer:
Pseudomembranous colitis is caused by Clostridium difficile, particularly following disruption of normal gut microbiota by antibiotics. Toxic megacolon and pneumocystis intestinalis are conditions associated with ulcerative colitis, an inflammatory bowel disease affecting the colon and rectum.
Step-by-step explanation:
The bacterium known for causing pseudomembranous colitis is Clostridium difficile (C. difficile). This condition often occurs after the normal gut microbiota is disrupted by antibiotics. The pseudomembranous colitis is a result of the bacterium producing two primary toxins - toxin A (TcdA) and toxin B (TcdB). These toxins act to inactivate small GTP-binding proteins within the colon, leading to cell death, focal necrosis, and ulceration that can progress to the more severe pseudomembranous colitis, which is characterized by inflammation of the colon and the development of a pseudomembrane made up of fibrin, dead epithelial cells, and leukocytes. Patients at increased risk of C. difficile infection include those who are immunocompromised, those with a history of long-term antibiotic use, and those in healthcare settings.
On the other hand, toxic megacolon and pneumocystis intestinalis are conditions associated with ulcerative colitis, an inflammatory bowel disease (IBD) that causes inflammation and ulcers in the colon and rectum. Though pseudomembranous colitis can be associated with the use of antibiotics in patients with ulcerative colitis, it is specifically C. difficile that causes the pseudomembranous colitis.