Sigmoid volvulus

A 90-year-old woman with sigmoid volvulus.
(A) Drawing shows X-marks-the-spot sign caused by crossing transition points. Horizontal lines and schematic axial insets indicate levels of B and C. Oblique lines in insets indicate two transition zones oriented in opposite directions, producing an X shape.
(B) Contrast-enhanced axial computed tomography (CT) image through proximal-superior transition (arrows).
(C) Axial CT image 15 mm caudal to B through distal-inferior sigmoid transition (arrows), which is oriented in the direction opposite to the proximal transition.