American Board of Internal Medicine (ABIM) Certification Practice Exam

Which organism is most commonly identified as a cause of diarrhea in the United States?

1. Giardia lamblia

2. Amebiasis

3. Clostridium difficile

4. Cryptosporidium parvum

The correct answer is: Giardia lamblia

The organism most commonly identified as a cause of diarrhea in the United States is Clostridium difficile. This bacterium is typically associated with antibiotic use, where it can overgrow in the gut flora and lead to infections characterized by severe diarrhea, and in some cases, colitis. C. difficile infections have become increasingly prevalent, especially in healthcare settings. Giardia lamblia is indeed a significant cause of diarrhea, particularly in regions with poor sanitation and among travelers to endemic areas. However, its overall incidence in the general population of the United States is lower compared to C. difficile, especially in the context of outbreaks and hospital-associated infections. Amebiasis, caused by Entamoeba histolytica, is less common in the U.S. and typically seen in travelers returning from endemic areas rather than as a frequent cause of diarrhea in the general population. Cryptosporidium parvum can also lead to diarrhea, especially in immunocompromised individuals, but its frequency is not as high as C. difficile in the general population. Therefore, based on the context of commonly occurring infectious causes of diarrhea in the United States, Clostridium difficile is recognized as the leading organism associated with these cases.