A 60 year old woman presented to OPD with dysphagia. No history of weight loss or heartburn. No change in bowel habits. While doing endoscopy there is some difficulty passing through the useful investigation?

A 60 year old woman presented to OPD with dysphagia. No history of weight loss or heartburn. No change in bowel habits. While doing endoscopy there is some difficulty passing through the LES (lower esophageal sphincter), but no other abnormality is noted. What is the single most useful investigation?