**Core Concept:** Amoebiasis is a parasitic infection caused by the protozoan Entamoeba histolytica, which primarily affects the large intestine. The infection can lead to symptoms like diarrhea, abdominal pain, and fever. The infection is transmitted via contaminated food or water, and the cyst form of the parasite can survive outside the body for weeks, allowing for its spread.

**Why the Correct Answer is Right:** The most common site for Amoebiasis is the large intestine (colon) where the trophozoites of E. histolytica multiply and cause inflammation and tissue damage. The infection leads to symptoms due to the inflammation and tissue destruction.

**Why Each Wrong Option is Incorrect:**

A. Liver (Option A): E. histolytica primarily affects the large intestine, not the liver. While liver abscesses can occur as a complication, they are less common compared to intestinal involvement.

B. Small intestine (Option B): Although E. histolytica can invade the small intestine, the primary site of infection is the large intestine.

C. Mouth (Option C): E. histolytica does not typically infect the mouth, as the parasite requires a warm, moist environment to thrive, which is not present in the oral cavity.

D. Spleen (Option D): While splenic amoebic abscesses can occur, they are less common than intestinal or liver involvement.

**Clinical Pearl:** Proper handwashing and sanitation can help prevent the transmission of E. histolytica, as the cyst form can survive outside the body for weeks, allowing for contamination of food and water sources. In endemic regions, it is essential to address the issue of contaminated water sources and improve sanitation to reduce the incidence of amoebiasis.