For a patient with foul-smelling diarrhea who recently returned from Borneo, what is the most likely causative organism?

In cases of foul-smelling diarrhea, particularly in a patient who has recently traveled to a region like Borneo, Giardia intestinalis emerges as the most probable causative organism. Giardia is a protozoan parasite that can lead to giardiasis, which frequently presents with symptoms of greasy, foul-smelling stools due to malabsorption of fats. This characteristic stool quality aligns closely with the symptom described in the question.

Travel to tropical or subtropical regions where there is potential exposure to contaminated water or food elevates the risk for giardiasis. The life cycle of Giardia involves cysts that can survive in water, making it easy to contract the infection from drinking untreated water or consuming contaminated food.

While Campylobacter jejuni can lead to diarrhea, it typically results in more watery stools and bloody diarrhea but not specifically foul-smelling. Cryptosporidium parvum may cause diarrhea, especially in immunocompromised individuals, but the stool often is not described as foul-smelling in the same manner as giardiasis. Escherichia coli O157 primarily causes bloody diarrhea and has a different presentation, typically associated with undercooked beef or contaminated food rather than foul-smelling diarrhea linked to travel exposure.

Therefore, Giardia intestinalis