<prompt> Mexican hat sign is seen inSigmoid volvulusMidgut volvulusSessile colonic polypPedunculated polyp  <response>: 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 