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The coffee bean sign is characteristic of sigmoid volvulus and is seen on abdominal X-rays. This sign appears as a dilated loop of the sigmoid colon that resembles a coffee bean due to its shape and the presence of gas within the bowel. It indicates the presence of colonic obstruction caused by the twisting of the sigmoid colon, which can compromise blood flow and lead to potential bowel ischemia if not addressed promptly.

The coffee bean appearance arises when the distended colon presents a prominent central air-filled lumen, with the soft tissue outlines of the bowel creating the distinctive shape. This finding is crucial for diagnosis, as it helps differentiate sigmoid volvulus from other causes of bowel obstruction.

Other signs mentioned in the options refer to different conditions: the double bubble sign suggests duodenal atresia, the string of pearls sign indicates small bowel obstruction, and free air under the diaphragm is associated with perforation of a hollow organ. None of these options pertain specifically to the sigmoid colon's twisting, underscoring the importance of recognizing the coffee bean sign in the context of sigmoid volvulus diagnosis.