A 42-year-old man presents with long-standing abdominal pain after meals, which is relieved by over-the-counter antacids. The patient has lost 9 kg (20 lb) in the past year. Physical examination reveals peripheral edema and ascites. Laboratory studies show decreased serum albumin but normal serum levels of transaminases and gastrin. Gross and microscopic examination of this patient's stomach would most likely show which of the following pathologic changes?