A 55-year-old woman comes to the physician because of recurrent chest pain over the past 3 years. These episodes are not related to activity or exertion. She has the sensation that food gets stuck in her chest. She has occasional nocturnal coughs and regurgitation. A chest X-ray during an episode shows widened mediastinum. There are no other bowel problems and no abdominal pain She is given a trial of proton pump inhibitors (PPI) but there is no relief. Which of the following is the best test to confirm the diagnosis of the underlying disease?