A patient who has returned from South America with symptoms of watery diarrhea and a low-grade fever may be infected with which organism?

The symptoms of watery diarrhea and a low-grade fever, combined with the patient's recent travel to South America, strongly suggest an infection with Vibrio cholerae. This organism is known to cause cholera, which is characterized by profuse watery diarrhea, often described as "rice-water stools," and can also lead to dehydration and electrolyte imbalances if untreated.

Cholera is endemic in many parts of South America, particularly in areas with poor sanitation or where there has been a recent outbreak. Thus, travel history to this region, accompanied by the described symptoms, makes Vibrio cholerae the most likely pathogen involved.

In contrast, while Escherichia coli can cause diarrhea, particularly enterotoxigenic E. coli strains, it typically presents with more symptoms such as cramping and may not fit the specific profile of travel-related cholera. Campylobacter jejuni is primarily associated with undercooked poultry and may lead to more bloody diarrhea, while Yersinia enterocolitica can cause gastrointestinal symptoms but is less common and often linked to certain foods like pork.