## Core Concept
Traveller's diarrhoea is a common condition affecting individuals who travel to areas with different sanitation and hygiene standards. It is primarily caused by consuming contaminated food or water. The condition is often associated with bacterial infections.

## Why the Correct Answer is Right
The correct answer, **Enterotoxigenic Escherichia coli (ETEC)**, is the most common cause of traveller's diarrhoea. ETEC produces enterotoxins that lead to increased secretion of fluid and electrolytes into the intestinal lumen, causing watery diarrhoea. This bacterium is commonly found in contaminated food and water, especially in areas with poor sanitation.

## Why Each Wrong Option is Incorrect
* **Option A:** While *Salmonella* can cause diarrhoea, it is not the most common cause of traveller's diarrhoea. *Salmonella* infections are more commonly associated with food poisoning.
* **Option B:** *Shigella* species can cause diarrhoea, particularly in areas with poor hygiene. However, they are not as commonly associated with traveller's diarrhoea as ETEC.
* **Option D:** *Vibrio cholerae* causes cholera, a severe diarrhoeal disease usually associated with contaminated water. While it can affect travellers, it is not the most common cause of traveller's diarrhoea.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the risk of traveller's diarrhoea can be reduced by following proper food and water safety precautions, such as drinking bottled or boiled water and avoiding raw or undercooked foods.

## Correct Answer: C. Enterotoxigenic Escherichia coli (ETEC).