**Core Concept**
Traveller's diarrhoea is a common gastrointestinal illness affecting individuals who travel abroad, particularly to areas with poor sanitation and hygiene. The condition is primarily caused by an infectious agent that contaminates food or water.

**Why the Correct Answer is Right**
The most common sensitive causative organism for Traveller's diarrhoea is a bacterium that adheres to the intestinal mucosa, leading to inflammation and fluid loss. The organism secretes toxins that cause the small intestine to secrete excess fluid, resulting in diarrhoea. The pathogenesis involves the toxin-mediated alteration of the intestinal tight junctions, allowing chloride and water to flow into the intestinal lumen.

**Why Each Wrong Option is Incorrect**
**Option A:** Other common causative organisms for Traveller's diarrhoea include *Campylobacter*, *Shigella*, and *Salmonella*, which are less sensitive and more invasive.
**Option B:** *Entamoeba histolytica* is a protozoan parasite that causes amoebic dysentery, a different condition characterized by bloody diarrhoea and abdominal pain.
**Option C:** *Giardia lamblia* is another protozoan parasite that causes giardiasis, a condition characterized by watery diarrhoea and malabsorption.

**Clinical Pearl / High-Yield Fact**
The most effective prevention strategy for Traveller's diarrhoea involves vaccination against *E. coli* and adherence to strict food and water safety guidelines, such as avoiding undercooked meat, unpasteurized dairy products, and untreated water.

**Correct Answer: A.** *Escherichia coli* (E. coli)