**Core Concept**
Appendicectomy, or the surgical removal of the appendix, has been associated with a reduced risk of certain gastrointestinal diseases. This protective effect is thought to be related to the role of the appendix in the development and maintenance of gut-associated lymphoid tissue (GALT).

**Why the Correct Answer is Right**
The appendix is believed to serve as a reservoir for beneficial gut bacteria, which are essential for the proper functioning of the immune system. By removing the appendix, the risk of certain gastrointestinal infections and diseases may be reduced. This is because the appendix may play a role in the development of the gut microbiome, particularly in early childhood.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the protective effect of appendicectomy is not primarily related to the prevention of constipation.
* **Option B:** This option is incorrect because the appendix is not primarily involved in the digestion of fatty foods.
* **Option C:** This option is incorrect because the protective effect of appendicectomy is not primarily related to the prevention of appendicitis recurrence.

**Clinical Pearl / High-Yield Fact**
The appendix has been referred to as a "vestigial organ," meaning that it has lost its original function through evolution. However, its role in the development and maintenance of the gut microbiome highlights its importance in the functioning of the immune system.

**Correct Answer: A. Ulcerative Colitis**