A 29-year-old African American woman comes to the emergency department because of chest pain and recurrent regurgitation of food. She has difficulty trying to swallow food and water. Physical examination shows significant weight loss, 7-kg (15-lb), since her last visit, six months ago. Esophageal manometry shows incomplete lower esophageal sphincter in response to swallowing. Results of barium swallow are obtained and shown below. Which of the following is most likely the diagnosis?