**Core Concept**
Lower small intestine obstruction typically presents with symptoms such as abdominal pain, vomiting, and constipation due to the accumulation of fluid and gas in the intestinal lumen. The diagnosis often involves a combination of clinical assessment, imaging studies, and laboratory tests to confirm the presence and location of the obstruction.

**Why the Correct Answer is Right**
The correct answer is **C. Plain abdominal X-ray with the 'coffee bean' sign**. This imaging modality is particularly useful in diagnosing lower small intestine obstructions, as it can reveal the presence of air-fluid levels and the characteristic 'coffee bean' sign, which is indicative of an obstruction in the distal small intestine. The 'coffee bean' sign is formed by the overlap of the air-filled colon and the dilated small intestine.

**Why Each Wrong Option is Incorrect**
**Option A:** Upper GI endoscopy is not the best way to diagnose lower small intestine obstruction, as it is primarily used for visualizing the upper gastrointestinal tract.

**Option B:** CT scan is a sensitive and accurate imaging modality for diagnosing intestinal obstructions, but it is not the best initial choice due to its higher cost and radiation exposure compared to plain abdominal X-ray.

**Option D:** Colonoscopy is not indicated in the diagnosis of lower small intestine obstruction, as it is primarily used for evaluating the colon and rectum.

**Clinical Pearl / High-Yield Fact**
The 'coffee bean' sign on plain abdominal X-ray is a classic finding in lower small intestine obstruction and should prompt further investigation to determine the cause of the obstruction.

**Correct Answer: C. Plain abdominal X-ray with the 'coffee bean' sign**