**Core Concept**
Peptic ulcer perforation is a life-threatening complication of peptic ulcer disease, characterized by a sudden breach in the mucosal lining of the stomach or duodenum, leading to leakage of gastric contents into the peritoneal cavity. The investigation of choice is crucial in diagnosing the condition and guiding further management.

**Why the Correct Answer is Right**
Barium meal and follow-through (Option C) is the investigation of choice in peptic ulcer perforation. This diagnostic test involves the ingestion of a barium sulfate suspension, which outlines the gastrointestinal tract on X-rays. In the setting of a perforation, barium will leak into the peritoneal cavity, allowing for visualization of the perforation site on imaging studies. This is particularly useful in diagnosing perforations that are not yet evident on plain abdominal X-rays.

**Why Each Wrong Option is Incorrect**
* **Option A:** Upper GI endoscopy is not the investigation of choice in peptic ulcer perforation, as it may exacerbate the perforation and is not suitable for evaluating free air or fluid in the peritoneal cavity.
* **Option B:** CT scan of the abdomen is useful in evaluating complications of peptic ulcer disease, but it is not the initial investigation of choice for diagnosing perforation.

**Clinical Pearl / High-Yield Fact**
The "cup and saucer" sign on a plain abdominal X-ray is a classic radiographic finding in peptic ulcer perforation, representing the air-filled stomach and the fluid-filled peritoneal cavity.

**Correct Answer: C. Barium meal and follow-through.**