**Core Concept**
Diffuse esophageal spasm (DES) is a condition characterized by abnormal muscle contractions of the esophagus, leading to chest pain and dysphagia. The investigation of choice should be able to accurately diagnose and quantify the extent of these muscle contractions.

**Why the Correct Answer is Right**
The investigation of choice in diffuse esophageal spasm is high-resolution manometry (HRM). HRM measures the pressure and coordination of the esophageal muscles as a person swallows. It can detect abnormal muscle contractions, such as those seen in DES, and provide information on the esophageal pressure topography. This information helps in diagnosing and differentiating between various esophageal disorders.

**Why Each Wrong Option is Incorrect**
**Option A:** Esophageal manometry is a related but older technology that provides less detailed information than HRM. While it can detect some abnormalities, it is not as sensitive or specific as HRM for diagnosing DES.

**Option B:** Upper GI series (barium swallow) can show some abnormalities in the esophagus, such as a "corkscrew" appearance, but it is not the investigation of choice for DES. It is more useful for diagnosing structural abnormalities, such as esophageal strictures or tumors.

**Option C:** Endoscopy is primarily used to visualize the mucosal surface of the esophagus and can diagnose conditions such as esophagitis or Barrett's esophagus. While it may show some abnormalities in the esophagus, it is not the investigation of choice for DES.

**Option D:** Esophageal pH monitoring is used to diagnose gastroesophageal reflux disease (GERD) and measure acid reflux in the esophagus. It is not relevant to the diagnosis of DES.

**Clinical Pearl / High-Yield Fact**
When considering the investigation of choice for diffuse esophageal spasm, remember that HRM provides a more detailed and accurate assessment of esophageal muscle function than other modalities. This is essential for diagnosing and managing DES effectively.

**Correct Answer: C. Esophageal manometry.