A 68-year-old man from Birmingham, England, has had "heartburn" and substernal pain after meals for 25 years. For the past year, he has had increased pain with difficulty swallowing both liquids and solids. On physical examination, there are no remarkable findings. Upper gastrointestinal endoscopy shows an ulcerated lower esophageal mass that nearly occludes the lumen of the esophagus. A biopsy specimen of this mass is most likely to show which of the following neoplasms?

Adenocarcinomas of the esophagus are typically located in the lower esophagus, where Barrett esophagus develops at the site of long-standing gastroesophageal reflux disease. Barrett esophagus is associated with an increased risk of developing adenocarcinoma, particularly when high-grade dysplasia is present. Columnar metaplasia may progress to dysplasia, then adenocarcinoma. Carcinoid tumors occur in different parts of the gut, including the appendix, ileum, rectum, stomach, and colon. Leiomyosarcoma of the esophagus is rare and is unrelated to a history of heartburn. Malignant lymphomas of the gastrointestinal tract do not commonly occur in the esophagus and are not related to reflux esophagitis. Squamous cell carcinomas of the esophagus are most often associated with a history of chronic alcoholism and smoking.