The apple core sign, also known as a napkin ring sign (bowel), is most frequently associated with constriction of the lumen of the colon by a stenosing annular colorectal carcinoma. Differential diagnosis The appearance of the apple-core lesion of the colon also can be caused by other diseases like, lymphoma with colonic involvement - appears more diffuse Crohn disease chronic ulcerative colitis ischemic colitis chlamydia infection colonic tuberculosis helminthoma colonic amoebiasis colonic cytomegalovirus villous adenoma radiosurgery, such as high doses of Cyberknife used for treating unresectable abdominal malignancies