In a 27 yr old male most common cause of a colo-vesical fistula would be:

Diveicular disease is the most common cause of colovesical fistula, followed by colon cancer and Crohn disease. Diveicular disease and colon cancer are rare in people younger than 40 years. Here this patient is young; hence the most suitable answer is Crohn's disease. Ref: ACP Medicine: Volume 1, 2007, Page 816.