**Core Concept**
Diagnosis of duodenal ulcers involves visualization of the upper gastrointestinal tract to identify mucosal breaks, ulcers, or other abnormalities. The chosen investigation should be sensitive and specific for detecting these lesions.

**Why the Correct Answer is Right**
Upper GI endoscopy is the most accurate diagnostic test for duodenal ulcers. It involves passing a flexible endoscope through the mouth and into the stomach and duodenum to visually inspect the mucosa. This allows for direct visualization of ulcers, bleeding points, and other lesions, as well as biopsy sampling for histological examination. Endoscopy is particularly useful for identifying complications such as bleeding, perforation, or stricture formation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hypotonic duodenography is a radiographic investigation that uses barium to visualize the duodenum, but it is less sensitive and specific than endoscopy for detecting duodenal ulcers.
* **Option B:** Barium meal is a radiographic investigation that visualizes the upper gastrointestinal tract, but it is less sensitive than endoscopy for detecting small ulcers and may not provide direct visualization of the mucosa.
* **Option C:** Barium swallow is a radiographic investigation that visualizes the esophagus, but it is not suitable for diagnosing duodenal ulcers.
* **Option E:** Abdominal X-ray erect view is not a suitable investigation for diagnosing duodenal ulcers, as it does not provide sufficient detail of the upper gastrointestinal tract.

**Clinical Pearl / High-Yield Fact**
Endoscopy is not only diagnostic but also therapeutic, allowing for the removal of ulcers, bleeding points, and other lesions, as well as the placement of stents or clips to manage complications.

**Correct Answer: D. Upper GI endoscopy.**