Question: A 54-year-old man visited Chungbuk National University Hospital outpatient clinic with abnormal colonoscopic findings at an outside hospital. He had been taking antihypertensive drugs and had undergone surgery for duodenal ulcer perforation approximately 30 years ago. He was a current smoker with no significant recent medical problems. There were no abnormalities noted on a colonoscopy 3 years ago, and he underwent a screening colonoscopy at this time. Colonoscopy at our hospital showed multiple, variable-sized, round elevated lesions, with a smooth surface, noted in the ascending colon. They closely resembled subepithelial lesions (Fig. A). When the injection needle punctured and aspirated the lesions, they collapsed without releasing any fluid (Fig. B). CT of the abdomen showed multiple intramural air-containing cystic lesions in the ascending colon (Fig. C, arrows). We decided upon a conservative approach with only follow-up, because the patient was asymptomatic with no abnormalities noted on physical examination and laboratory studies. Colonoscopy and CT scan were performed a year later, which showed no significant changes in the multiple cystic lesions seen in the ascending colon. The patient continued to be asymptomatic with no abnormalities noted on physical examination and laboratory studies. What is the diagnosis of the multiple cystic lesions in the colon?