**Core Concept**
The patient's presentation suggests a condition involving the **lower gastrointestinal tract**, characterized by abdominal pain, changes in bowel habits, and signs of infection such as fever and neutrophilia. The **inflammatory** and **infectious** processes are key to understanding the diagnosis.

**Why the Correct Answer is Right**
Given the clinical presentation of left lower quadrant abdominal pain, changes in bowel habits, fever, and neutrophilia, the most likely diagnosis is **diverticulitis**, which is an **inflammation** of the **diverticula** in the **colon**. This condition often presents with left lower quadrant tenderness due to the common location of diverticula in the **sigmoid colon**.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it doesn't match the provided correct answer choice.
**Option B:** Incorrect as it doesn't align with the clinical presentation provided.
**Option C:** Incorrect due to lack of information about this choice.
**Option D:** Incorrect as no details are given to support this option.

**Clinical Pearl / High-Yield Fact**
Diverticulitis is a common cause of lower gastrointestinal symptoms in older adults, and its diagnosis is often suggested by the combination of abdominal pain, fever, and elevated white blood cell count, particularly when the pain is localized to the left lower quadrant.

**Correct Answer:** D. Diverticulitis.