This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Unknown
A patient with diarrhea has a fecal smear that is negative for leukocytes. The patient's diarrhea is most likely caused by 
Select and explain. 
A. Campylobacter enteritis, B. Shigellosis, C. Typhoid fever, D. Enterotoxigenic Escherichia coli
D: Enterotoxigenic Escherichia coli
Answer: D: Enterotoxigenic Escherichia coli 
Explanation: The enterotoxigenic strain of Escherichia coli secretes a heat-labile toxin that stimulates adenylate cyclase, producing a cholera-like syndrome that is responsible for the majority of cases of traveler's diarrhea. Because the bowel mucosa is not invaded and there is no toxin-induced damage, the fecal smear is negative for leukocytes. Campylobacter enteritis, shigellosis, ty­phoid fever, and amebiasis are all invasive, and infected individuals have stools with inflammatory cells. Ty­phoid fever is unique because the inflammatory cells are not neutrophils, but mononuclear cells.