Which organism is the leading cause of antibiotic-associated diarrhea and pseudomembranous colitis?

• A. Staphylococcus aureus
• B. Clostridioides difficile
• C. Escherichia coli O157:H7
• D. Salmonella Typhi

Answer: (B)

Antibiotics can wipe out the normal gut bacteria, which lets Clostridioides difficile overgrow. As it multiplies, C. difficile releases toxins that injure the colon’s lining, triggering diarrhea and the formation of pseudomembranes seen in pseudomembranous colitis. This combination—diarrhea following antibiotic use plus characteristic colonic injury from the toxins—is what makes this organism the most common cause of antibiotic-associated diarrhea and pseudomembranous colitis. Other organisms listed cause different illnesses: Staphylococcus aureus is more tied to toxin-mediated food poisoning or other infections, E. coli O157:H7 typically causes hemorrhagic colitis, and Salmonella Typhi leads to typhoid fever, not the classic antibiotic-associated pseudomembranous colitis.