## **Core Concept**
Traveller's diarrhoea is a condition characterized by the sudden onset of loose stools and abdominal cramps during or shortly after travel, often due to consuming contaminated food or water. The primary cause is bacterial infection. **Escherichia coli** is a common pathogen implicated in this condition.

## **Why the Correct Answer is Right**
The correct answer, **E. coli (specifically Enterotoxigenic E. coli or ETEC)**, is right because ETEC is a well-documented cause of traveller's diarrhoea. It produces heat-labile and heat-stable toxins that lead to increased secretion of fluids and electrolytes into the intestine, causing the diarrhoea. This mechanism involves the **LT (heat-labile toxin) and ST (heat-stable toxin)** of ETEC, which activate adenylate cyclase and guanylate cyclase, respectively, in the intestinal epithelial cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain strains of **Salmonella** can cause diarrhoea, they are more commonly associated with enteric fever (typhoid and paratyphoid) or gastroenteritis rather than specifically traveller's diarrhoea.
- **Option B:** **Shigella** causes dysentery, which is characterized by bloody diarrhoea and is not the most common cause of traveller's diarrhoea.
- **Option D:** **Vibrio cholerae** causes cholera, a severe diarrhoeal disease usually associated with contaminated water, but it is not the most typical cause of traveller's diarrhoea in most regions.

## **Clinical Pearl / High-Yield Fact**
A memorable point for exams is that **traveller's diarrhoea prophylaxis** often includes **Rifaximin**, a non-absorbable antibiotic, and **Azithromycin** for treatment. Prevention measures include avoiding unpeeled fruits, unpasteurized dairy products, and untreated water.

## **Correct Answer:** C. E. coli.