A 56-year-old female with a penicillin allergy and hives has been taking clindamycin (Cleocin®) for recurrent tooth pain. She now presents with diarrhea and stomach aches every day. Gross decay is noted on tooth #30. Infection with which organism is the most likely cause of her intestinal symptoms?

Correct answer: A. Clostridioides difficile

• A. CORRECT. Clindamycin is the antibiotic most classically associated with Clostridioides difficile (C. diff) colitis. Clindamycin disrupts normal gut flora, allowing C. difficile to overgrow and produce toxins that cause pseudomembranous colitis, manifesting as profuse watery diarrhea, abdominal cramps, and in severe cases, fever and bloody stools. This is a well-established and clinically important association.
• B. INCORRECT. Salmonella enterica causes foodborne gastroenteritis typically from contaminated poultry or eggs. It is not related to antibiotic use and would not explain symptoms developing in the context of prolonged clindamycin therapy.
• C. INCORRECT. Escherichia coli can cause various GI infections (e.g., traveler's diarrhea, hemolytic uremic syndrome), but these are typically food/water-borne, not antibiotic-associated in this context.
• D. INCORRECT. Lactobacillus acidophilus is a beneficial probiotic organism. Its overgrowth does not cause diarrhea; in fact, it is often used to prevent antibiotic-associated diarrhea by restoring normal flora.