This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Gastrointestinal Radiology, Subject name: Radiology
[{'content': 'Mexican hat sign is seen in \nSelect and explain. \nA. Sigmoid volvulus, B. Midgut volvulus, C. Sessile colonic polyp, D. Pedunculated polyp', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Pedunculated colonic polyps form the "Mexican hat sign." The Mexican hat sign is formed by the appearance of 2 concentric rings; the outer ring represents the "en face" visualization of barium coating the surface of the head of a pedunculated polyp, while the inner ring represents a meniscus of barium surrounding the stalk of the polyp visualized through the head', 'role': 'assistant'}]