**Core Concept**
The diagnosis and management of intestinal obstruction involve a combination of clinical evaluation, laboratory tests, and imaging studies to determine the cause and location of the obstruction. Intestinal obstruction can be caused by mechanical or non-mechanical factors, and the choice of investigation depends on the suspected cause and location of the obstruction.

**Why the Correct Answer is Right**
The correct answer is a contrast enema, which is a radiographic examination that uses a barium or water-soluble contrast agent to visualize the intestinal lumen and identify the site and cause of obstruction. A contrast enema is particularly useful for identifying intestinal obstructions caused by intussusception, volvulus, or fecal impaction. The contrast agent helps to outline the bowel lumen and identify any narrowing or distortion caused by the obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Blood tests, such as complete blood count (CBC) and liver function tests (LFTs), may be ordered to evaluate for signs of intestinal ischemia or necrosis, but they are not the primary investigation for intestinal obstruction.
**Option B:** An abdominal ultrasound may be used to evaluate for signs of intestinal obstruction, such as bowel dilation or wall thickening, but it is not the primary investigation.
**Option C:** A CT scan with contrast is a sensitive and specific imaging modality for diagnosing intestinal obstruction, but it is not the primary investigation.

**Clinical Pearl / High-Yield Fact**
A contrast enema should be avoided in patients with suspected intestinal perforation or significant bowel ischemia, as it can exacerbate these conditions.

**Correct Answer:** C. CT scan with contrast