**Core Concept**
Diffuse esophageal spasm (DES) is a condition characterized by abnormal, simultaneous contractions of the esophageal muscles, leading to chest pain and dysphagia. This condition is distinct from other esophageal disorders, such as achalasia, where the primary issue is a failure of the lower esophageal sphincter to relax.

**Why the Correct Answer is Right**
The correct modality for diagnosing DES is manometry, which assesses the pressure and coordination of esophageal contractions. During manometry, a catheter is inserted through the nose and advanced into the esophagus to measure the pressure and movement of the esophageal muscles. This allows for the identification of abnormal, simultaneous contractions that are characteristic of DES.

**Why Each Wrong Option is Incorrect**
**Option A:** Barium swallow study is primarily used to evaluate the anatomy of the esophagus and can show irregularities in the esophageal lumen, but it is not sensitive for detecting the abnormal muscle contractions characteristic of DES.

**Option B:** Upper endoscopy is useful for visualizing the esophageal mucosa and can help identify esophageal inflammation or ulcers, but it does not assess esophageal muscle function.

**Option C:** Chest X-ray may show nonspecific signs of esophageal disease, such as esophageal dilatation, but it is not a reliable method for diagnosing DES.

**Clinical Pearl / High-Yield Fact**
A key feature of DES is its association with chest pain that is often described as a "chest tightness" or "pressure," which can be exacerbated by eating or swallowing. This symptom can be difficult to distinguish from other causes of chest pain, such as cardiac disease.

**Correct Answer:** C. Manometry.