**Question:** Traveller's diarrhea is caused by

A. Salmonella
B. E. coli
C. Shigella
D. Campylobacter

**Core Concept:** Traveller's diarrhea is a common gastrointestinal disorder seen in individuals traveling to developing countries with inadequate sanitation and hygiene. It is typically characterized by watery diarrhea, abdominal cramps, and sometimes vomiting and fever.

**Why the Correct Answer is Right:** Traveller's diarrhea is mainly caused by the ingestion of contaminated food and water, leading to the colonization and infection of the gut with certain bacteria. In this case, the correct answer is D. Campylobacter.

Campylobacter species, particularly Campylobacter jejuni, are the most common cause of acute gastroenteritis, including traveller's diarrhea. These bacteria are often found in undercooked poultry, raw or undercooked meat, unpasteurized milk and dairy products, and contaminated water. The presence of Campylobacter in the gut results in inflammation of the intestinal lining, leading to the clinical symptoms of traveller's diarrhea.

**Why Each Wrong Option is Incorrect:**

A. Salmonella: Salmonella is another common cause of gastroenteritis but is typically associated with typhoid fever and is not the primary cause of traveller's diarrhea.

B. E. coli: Most Escherichia coli (E. coli) strains are harmless commensals in the human gut. While some E. coli strains can cause gastrointestinal infections, they are not the primary cause of traveller's diarrhea.

C. Shigella: Shigella is another bacterial agent causing gastroenteritis, particularly in children, but is less common in adults and not typically associated with traveller's diarrhea.

**Clinical Pearl:** To prevent traveller's diarrhea, it is essential to practice good hygiene, consume pasteurized food and water, and avoid undercooked meat and raw milk. In case of symptoms, oral rehydration therapy and antibiotics (if prescribed by a healthcare provider) are the primary treatments.