**Core Concept**
Irritable Bowel Syndrome (IBS) is a functional gastrointestinal disorder characterized by chronic abdominal pain, changes in bowel habits, and bloating in the absence of any organic cause. The pathophysiology of IBS is multifactorial, involving alterations in gut motility, hypersensitivity, and altered gut microbiota.

**Why the Correct Answer is Right**
The patient's symptoms of abdominal pain, constipation, and mucus in stools for four years, along with tenderness in the left iliac fossa, are consistent with IBS. The chronic nature of the symptoms and the absence of any organic cause make IBS a likely diagnosis. The presence of mucus in stools is a common feature of IBS, and the patient's symptoms of constipation and incomplete evacuation are also consistent with this diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Ulcerative Colitis (UC) is a chronic inflammatory bowel disease characterized by inflammation of the colon, which typically presents with diarrhea, bloody stools, and weight loss. The patient's symptoms of constipation and mucus in stools do not support this diagnosis.

**Option B:** Crohn's disease is another type of inflammatory bowel disease that can affect any part of the gastrointestinal tract. However, it typically presents with symptoms of diarrhea, weight loss, and abdominal pain, which do not match the patient's symptoms.

**Option C:** Diverticulitis is an inflammation of the diverticula in the colon, which can present with symptoms of abdominal pain, fever, and altered bowel habits. However, the patient's symptoms of constipation and mucus in stools do not support this diagnosis.

**Clinical Pearl / High-Yield Fact**
IBS is a common condition that affects up to 20% of the population, and its diagnosis is often based on the Rome IV criteria, which includes symptoms of abdominal pain, changes in bowel habits, and the absence of any organic cause.

**Correct Answer: C. Diverticulitis**