DEAR DR. BLONZ: I will be having a routine colonoscopy next month and just received the preparation instructions. It says that in the week before the test, I should avoid seeds and even corn (one of my favorite foods). : A colonoscopy is a test in which a specialist, usually a gastroenterologist, uses a device that provides a view of the lining of the large intestine. It is a method used both for routine screening and for patients with symptoms that require a closer look. The preparation during the days before the test is no fun, but this is a very important procedure that can be a lifesaver.