Which of the following findings on an abdominal X-ray suggests an obstruction?

Air-fluid levels in bowel loops on an abdominal X-ray are indicative of an obstruction. This finding is a result of the accumulation of fluid and gas in the bowel due to a blockage, preventing normal passage and resulting in distension of the bowel loops. The presence of these air-fluid levels typically suggests that there is an area of the intestine where contents are not moving efficiently, a hallmark of intestinal obstruction.

In contrast, the presence of free air usually indicates perforation of a hollow organ rather than obstruction. No bowel gas present might suggest severe obstruction or inactivity but does not provide clear evidence of an obstruction. An enlarged colon can occur in various conditions, including colonic obstruction, but by itself is not a definitive indicator without the associated air-fluid levels. This combination of findings is crucial for diagnosing the nature and severity of the obstruction in clinical practice.