If you’re preparing for the United States Medical Licensing Examination® (USMLE®) Step 2 exam, you might want to know which questions are most often missed by test-prep takers. Check out this example from Kaplan Medical, and read an expert explanation of the answer. Also check out all posts in this series.
This month’s stumper
A 50-year-old man returns to his home in Minnesota after a diving trip to Belize. The day after his return, he comes to the physician because of diarrhea, abdominal cramps, and nausea. His temperature is 37 °C (98.6 °F). His stools do not contain mucus or blood. Microscopic examination of a stool sample reveals no leukocytes.
Which of the following is the most likely pathogen?
A. Bacillus cereus.
B. Clostridium perfringens.
C. Escherichia coli.
D. Rotavirus.
E. Staphylococcus aureus.
The correct answer is C.
Kaplan Medical explains why
Traveling abroad often entails abrupt changes in diet and climate, as well as exposure to conditions of poor sanitation, all of which results in a high incidence of diarrhea. This is self-limiting and manifests with watery diarrhea and dehydration, but no fever or other signs of systemic infection. The most frequent cause of traveler’s diarrhea is enterotoxigenic Escherichia coli.
The remaining infectious agents listed here are all potential causes of noninflammatory diarrhea, which is not associated with blood and mucus in the stool, fever, systemic signs of infection, or fecal leukocytes.
Why the other answers are wrong
Choice A, B and E: Bacillus cereus, Clostridium perfringens, and Staphylococcus aureus, along with enterotoxigenic E. coli, are the most common agents associated with food poisoning due to production of toxins. All these pathogens produce a similar clinical picture of watery diarrhea, sometimes with nausea and vomiting, but no fever.
The history is key in differentiating them. Without much of a history, expect the pathogen to be E. coli. If refried rice is part of the history, think B. cereus. If custards or picnic-like food enters the picture, as well as immediate post-meal diarrhea, think S. aureus. Finally, C. perfringens will often be tested less about food poisoning and more of its classic complication of gas gangrene in a diabetic foot.
Choice D: Rotavirus is one of the most important infectious causes of diarrhea in infants and young children in developing countries. It may also cause diarrhea in adults exposed to infected children. Think crowded places like cruise ships as the source of the infection.
For more prep questions on USMLE Steps 1, 2 and 3, view other posts in this series.
The AMA and Kaplan have teamed up to support you in reaching your goal of passing the USMLE® or COMLEX-USA®. If you're looking for additional resources, Kaplan provides free access to tools for pre-clinical studies, including Kaplan’s Lecture Notes series, Integrated Vignettes, Shelf Prep and more.
