**Core Concept**
Diffuse esophageal spasm (DES) is a motility disorder characterized by abnormal, uncoordinated contractions of the esophageal smooth muscle. This results in chest pain, dysphagia, and regurgitation. The investigation of choice should be able to accurately diagnose and quantify these abnormal contractions.

**Why the Correct Answer is Right**
Manometry is the investigation of choice in diffuse esophageal spasm as it directly measures the pressure and coordination of esophageal contractions. High-resolution manometry (HRM) provides detailed information on esophageal motility and can accurately diagnose DES by identifying abnormal contractions, such as simultaneous and premature contractions. The distal contractile integral (DCI) and the 30 mmHg threshold are key parameters used to diagnose DES.

**Why Each Wrong Option is Incorrect**
**Option A:** Endoscopy is not the investigation of choice for DES as it cannot directly measure esophageal motility or diagnose abnormal contractions.
**Option B:** Barium swallow is not sensitive for detecting DES, as it relies on visualization of the esophagus and may not show abnormal contractions.
**Option C:** pH monitoring is used to diagnose gastroesophageal reflux disease (GERD) and is not relevant to the diagnosis of DES.

**Clinical Pearl / High-Yield Fact**
Remember that high-resolution manometry (HRM) is the gold standard for diagnosing esophageal motility disorders, including diffuse esophageal spasm.

**Correct Answer: C. pH monitoring is used to diagnose gastroesophageal reflux disease (GERD) and is not relevant to the diagnosis of DES.