The test with the highest diagnostic yield for detecting a colovesical fistula is:

E. Cystoscopy.

DISCUSSION: Of the tests listed above, cystoscopy provides the highest diagnostic yield, between 80% and 95%. The most common finding on cystoscopy is localized inflammation and bullous edema of the bladder mucosa. Actual demonstration of the fistula is unusual, no matter which test is utilized. Barium enema usually demonstrates some abnormality; however, precise delineation of the fistula occurs in only 30% of cases. Recently, CT has been shown to be useful in diagnosing colovesical fistula with accuracy approaching that of cystoscopy. The low diagnostic yields (20%) of cystography and colonoscopy have limited their use.