**Core Concept**
Barrett's esophagus is a pre-malignant condition characterized by the replacement of the normal squamous epithelium of the distal esophagus with metaplastic columnar epithelium. This condition is a complication of chronic gastroesophageal reflux disease (GERD) and increases the risk of esophageal adenocarcinoma.

**Why the Correct Answer is Right**
The diagnosis of Barrett's esophagus is primarily based on the visualization of columnar epithelium in the distal esophagus, typically at or below the squamo-columnar junction. This can be achieved through endoscopy, where the endoscopist looks for the presence of columnar-lined esophagus. Biopsies may also be taken for histological examination to confirm the diagnosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because endoscopy is not sufficient to diagnose Barrett's esophagus without the visualization of columnar epithelium. While endoscopy may be used to identify the presence of columnar-lined esophagus, it is not a definitive diagnostic tool.
**Option B:** This option is incorrect because radiographic imaging, such as barium swallow studies, may show signs suggestive of Barrett's esophagus, but it is not a reliable method for diagnosis. Biopsy and histological examination are still necessary for confirmation.
**Option C:** This option is incorrect because esophageal manometry is a diagnostic tool used to assess esophageal function and motility, not to diagnose Barrett's esophagus.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the presence of columnar-lined esophagus on endoscopy, combined with histological confirmation of intestinal metaplasia, is the gold standard for diagnosing Barrett's esophagus.

**Correct Answer:** C. Esophageal manometry is not used to diagnose Barrett's esophagus.