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A 68-year-old man from Birmingham, England, has had "heartburn"and substernal pain after meals for 25 years. For the past vear, he has had increased pain with difficulty swal- lowing 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?