• Recent instrumentation: Air enters the urinary bladder most commonly as a consequence of catheterization. The absence of any other abnormality favors that diagnosis. This patient was catheterized in the emergency room to obtain a clean urine sample.

• Fistula with the bowel: Gas in the urinary bladder lumen in the absence of recent instrumentation is diagnostic of a fistula with the bowel. Depending on the site of the fistula and the communicating portion of the bowel, a localizing sign such as focal bladder wall thickening, bowel wall thickening, or a mass and fat stranding would be expected.