 Penetrating disease creates abnormal passageways (fistulae) between the bowel and other structures, such as the skin.Inflammatory disease (or nonstricturing, nonpenetrating disease) causes inflammation without causing strictures or fistulae.A colonoscopy is the best test for making the diagnosis of Crohn's disease, as it allows direct visualization of the colon and the terminal ileum, identifying the pattern of disease involvement.