**Core Concept**
The patient's symptoms of epigastric pain radiating to the back suggest a diagnosis of peptic ulcer disease (PUD), specifically a duodenal ulcer. The investigation of choice for suspected PUD involves imaging to visualize the duodenum and stomach.

**Why the Correct Answer is Right**
The investigation of choice for suspected duodenal ulcer is an upper gastrointestinal (UGI) endoscopy. This is because endoscopy allows for direct visualization of the mucosa, enabling the identification of ulcers, erosions, and other mucosal abnormalities. Additionally, endoscopy provides an opportunity for biopsy and collection of tissue samples for histological examination. The UGI endoscopy is also useful for detecting complications of PUD, such as bleeding or perforation.

**Why Each Wrong Option is Incorrect**
**Option A:** An abdominal X-ray is not the investigation of choice for suspected duodenal ulcer. While an X-ray may show signs of perforation, such as free air under the diaphragm, it is not sensitive for detecting ulcers or other mucosal abnormalities.

**Option B:** A CT scan of the abdomen may be useful in detecting complications of PUD, such as bleeding or perforation, but it is not the investigation of choice for suspected duodenal ulcer. CT scans are more expensive and expose patients to higher radiation doses than endoscopy.

**Option C:** A barium swallow study may show signs of duodenal ulcer, such as a "double-density" sign, but it is less sensitive than endoscopy for detecting ulcers and other mucosal abnormalities.

**Clinical Pearl / High-Yield Fact**
A classic clinical correlation for duodenal ulcers is the presence of a "sentinel bleed," which is a small amount of bleeding that occurs at the edge of the ulcer and is often accompanied by a "black tarry" stool.

**Correct Answer: D. UGI Endoscopy**