**Core Concept**
Tuberculosis (TB) of the ileocecum is a form of extrapulmonary TB that occurs in the terminal ileum and cecum. The ileocecum is a common site for TB due to its high concentration of lymphoid tissue, including Peyer's patches, which are susceptible to TB infection.

**Why the Correct Answer is Right**
The ileocecum is the most common site involved in ileocaecal TB due to its rich blood supply and high lymphoid tissue concentration. The terminal ileum is particularly prone to TB infection due to its exposure to luminal bacteria, which can be ingested and cause an immune response. The cecum's proximity to the terminal ileum also makes it a common site for TB spread.

**Why Each Wrong Option is Incorrect**
**Option A:** The ascending colon is not the most common site involved in ileocaecal TB, although it can be affected.
**Option B:** The transverse colon is not typically involved in ileocaecal TB, which primarily affects the terminal ileum and cecum.
**Option C:** The appendix is a rare site for TB, and its involvement typically occurs as a result of adjacent ileocaecal TB spread.

**Clinical Pearl / High-Yield Fact**
In ileocaecal TB, the terminal ileum is often involved, and the disease can mimic other conditions such as Crohn's disease or appendicitis. A high index of suspicion and careful imaging studies are necessary for accurate diagnosis.

**Correct Answer:** C. The appendix is not the commonest site involved in ileocaecal T.B.