A 38-year-old woman has had nausea for 6 months. She reports no vomiting or diarrhea. On physical examination, there are no remarkable findings. Upper gastrointestinal endoscopy shows diffuse gastric mucosal erythema with focal mucosal erosions, but no ulcerations. The esophageal and duodenal mucosal surfaces appear normal. Gastric biopsies are obtained and microscopic examination shows focal mucosal hemorrhage, loss of the surface epithelium, and increased numbers of neutrophils, lymphocytes, and plasma cells in an edematous mucosa. No Helicobacter pylori organisms are seen. Laboratory studies show a normal serum gastrin level. Which of the following pharmacologic agents is most likely to produce these findings?