A 53-year-old woman has had nausea, vomiting, and mid-epigastric pain for 5 months. On physical examination, there are no significant findings. An abdominal CT scan shows gastric outlet obstruction. Upper gastrointestinal endoscopy shows an ulcerated 2x4 cm bulky mass in the antrum at the pylorus. A urease test is positive. Which of the following neoplasms is most likely to be seen in a biopsy specimen of this mass?