**Core Concept**
The question requires knowledge of the pathophysiology and clinical presentation of amoebiasis, a parasitic infection caused by *Entamoeba histolytica*. The correct answer is related to the anatomical site where the amoeba typically infects the human gut.

**Why the Correct Answer is Right**
The commonest site of amoebiasis in the gut is the caecum, where the amoeba *Entamoeba histolytica* attaches to the mucosal lining and invades the intestinal tissue. This leads to the characteristic symptoms of amoebic dysentery, including diarrhea, abdominal pain, and weight loss. The caecum's rich blood supply and high concentration of mucus provide an ideal environment for the amoeba to multiply and cause infection.

**Why Each Wrong Option is Incorrect**
* **Option A:** The ileum is not a common site for amoebiasis, as the amoeba typically infects the large intestine. The ileum's role in absorbing nutrients and its less mucus-rich environment make it less susceptible to amoebic infection.
* **Option C:** The ascending colon is not typically involved in amoebiasis, as the infection usually starts in the caecum and then spreads to the rest of the large intestine.
* **Option D:** The transverse colon is not a common site for amoebiasis, as the infection usually affects the more proximal parts of the large intestine.
* **Option E:** The sigmoid colon is not the most common site for amoebiasis, although it can be involved in advanced cases.

**Clinical Pearl / High-Yield Fact**
The presence of **pseudomebranous colitis**, characterized by a thick, grayish-yellow slough covering the mucosal surface, is a hallmark of amoebic dysentery. This clinical finding is often associated with a poor prognosis and requires prompt medical attention.

**Correct Answer:** B. Caecum