What is the most likely pathogen responsible for watery diarrhoea in a woman who just returned from Mexico?

The most likely pathogen responsible for watery diarrhoea in a woman who has recently returned from Mexico is Escherichia coli, particularly diarrhea-producing strains known as enterotoxigenic E. coli (ETEC). This pathogen is a common cause of traveler’s diarrhea and is frequently associated with water or food contaminated by feces, which can occur in regions where sanitation practices may not be optimal, such as certain areas in Mexico.

ETEC produces heat-labile and heat-stable enterotoxins that trigger gastrointestinal symptoms, including watery diarrhoea. The clinical presentation is often acute and can be accompanied by abdominal cramping and nausea, making it a frequent diagnosis in travelers returning from endemic areas.

In contrast, the other pathogens listed typically have different epidemiological distributions or clinical presentations. For example, Giardia intestinalis usually leads to a more prolonged diarrheal illness characterized by greasy stools rather than watery diarrhoea. Aeromonas hydrophila can cause gastroenteritis, but it is less commonly associated with international travel and is more often linked to freshwater exposure. Entamoeba histolytica is typically associated with dysentery and can present with bloody stools, distinguishing it from the watery diarrhoea caused by E. coli.