A 49-year-old woman has a history of peptic ulcer disease for which she has been treated with proton pump inhibitors. She has had nausea with vomiting for the past 2 months. Upper GI endoscopy reveals three circumscribed, round, smooth lesions in the gastric body from 1 to 2 cm in diameter. Biopsies are taken and microscopically show the lesions to consist of irregular glands that are cystically dilated and lined by flattened parietal and chief cells. No inflammation, Helicobacter pylori, metaplasia, or dysplasia is present. What is the most likely diagnosis?