University of Central Florida (UCF) MCB2004 Microbiology for Health Professionals Practice Exam 2

Antibiotic-associated diarrhea is primarily caused by the overgrowth of Clostridium difficile, particularly after a course of antibiotics that disrupt the normal flora of the gut. Antibiotics can eliminate a significant number of beneficial bacteria that help maintain a balanced gut environment. When these beneficial microorganisms are reduced, C. difficile, which often resides in the intestines in small numbers, can proliferate and produce toxins that lead to inflammation of the intestinal lining and diarrhea.

This mechanism highlights the importance of the gut microbiome in maintaining intestinal health. Conditions such as antibiotic-associated diarrhea are more likely to occur in individuals who have undergone antibiotic treatment, especially broad-spectrum antibiotics, which can indiscriminately affect a wide range of bacteria, including the commensal ones that usually keep opportunistic pathogens in check.

In addition to C. difficile, the other options presented in the question involve less common mechanisms or different types of infections that do not primarily arise from antibiotic use and the ensuing imbalance in gut flora. Therefore, the overgrowth of Clostridium difficile as a result of the disruption of normal gut flora is recognized as the main cause of antibiotic-associated diarrhea.