**Core Concept**
The investigation for small intestine abnormalities involves a combination of imaging modalities and functional tests to assess the structure and function of the small intestine. This includes radiographic studies, endoscopic procedures, and nuclear medicine tests.

**Why the Correct Answer is Right**
The correct investigation for small intestine abnormalities is **Enteroclysis**, also known as small bowel follow-through (SBFT). It is a radiographic study that involves the oral ingestion of a barium sulfate suspension, followed by serial radiographs taken at regular intervals to visualize the small intestine. This study is particularly useful for detecting abnormalities such as Crohn's disease, ulcerative colitis, and intestinal tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** **Upper GI Endoscopy** is primarily used to investigate the upper gastrointestinal tract, including the esophagus, stomach, and duodenum. While it can be used to visualize the proximal small intestine, it is not the primary investigation for small intestine abnormalities.

**Option B:** **Computed Tomography (CT) Enterography** is a radiographic study that uses CT technology to visualize the small intestine. However, it is not as sensitive as Enteroclysis for detecting small intestine abnormalities, particularly those involving the mucosa and submucosa.

**Option C:** **Magnetic Resonance Enterography (MRE)** is a non-invasive imaging modality that uses magnetic resonance technology to visualize the small intestine. While it is useful for detecting abnormalities such as Crohn's disease, it is not as widely available as Enteroclysis and may not be as sensitive for detecting small intestine abnormalities.

**Clinical Pearl / High-Yield Fact**
Remember that the choice of investigation for small intestine abnormalities depends on the specific clinical scenario and the type of abnormality suspected. A thorough history and physical examination are essential in guiding the selection of the most appropriate investigation.

**Correct Answer:** A. Upper GI Endoscopy