Bacteria causing antibiotic-associated diarrhea and colitis in older adults in hospitals.

• A. Escherichia coli
• B. Clostridium difficile
• C. Staphylococcus aureus
• D. Campylobacter jejuni

Answer: (B)

The main idea here is that antibiotic-associated colitis in older adults in hospitals is most commonly caused by Clostridioides difficile. When broad-spectrum antibiotics are given, the normal gut bacteria are suppressed, allowing C. difficile to overgrow and release toxins that inflame and damage the colon. This toxin-mediated process leads to diarrhea and, in some cases, pseudomembranous colitis, which is a classic scenario in elderly patients who recently received antibiotics in a hospital setting.

Understanding this helps distinguish it from other bacteria. Escherichia coli can cause diarrhea but is not the hallmark of antibiotic-associated colitis in the hospital. Staphylococcus aureus can cause various infections and some toxin-related GI symptoms, but it is not the typical cause of antibiotic-associated colitis in this context. Campylobacter jejuni is a common cause of bacterial gastroenteritis from food exposure rather than antibiotic-associated colitis in hospitalized elderly patients.

Key takeaway: recent antibiotic use in an elderly hospitalized patient with new diarrhea points most strongly to C. difficile as the responsible organism, due to its toxin-driven colitis and characteristic hospital-associated risk factors.