**Core Concept**
The patient's presentation suggests an acute abdomen, specifically diverticulitis given the location of the pain, constipation, and elevated leukocyte count. Diverticulitis is an inflammation of diverticula, small pouches in the wall of the colon, often caused by bacterial overgrowth and compromised blood supply.

**Why the Correct Answer is Right**
The correct answer is a CT scan with contrast, as it is the most sensitive and specific imaging modality for diagnosing diverticulitis. A CT scan helps identify the location, size, and extent of the inflammation, as well as any complications such as abscess formation. The use of contrast enhances the visualization of the bowel wall and surrounding structures.

**Why Each Wrong Option is Incorrect**
**Option A:** Ultrasound is not the best diagnostic study for diverticulitis as it is operator-dependent and may not provide a clear view of the entire colon. It may also not accurately diagnose complications like abscesses.

**Option B:** Plain abdominal radiographs are not useful for diagnosing diverticulitis as they do not provide enough detail to assess the bowel wall and surrounding structures.

**Option C:** Magnetic Resonance Imaging (MRI) can be used to diagnose diverticulitis, but it is not as readily available as CT scans and may not be as useful in emergency situations.

**Clinical Pearl / High-Yield Fact**
When evaluating acute abdominal pain, it's essential to consider the location of the pain to narrow down the differential diagnosis. In this case, left lower quadrant pain is highly suggestive of diverticulitis or other conditions like appendicitis.

**Correct Answer:** C. MRI
MRI is a useful imaging modality for diagnosing diverticulitis, though not the best among the given options.