A colonoscopy is a screening test for colorectal cancer and other intestinal issues. A colonoscopy lets your doctor examine the lining of your large intestine (colon) for abnormalities, with a thin flexible tube called a colonoscope. It has its own lens and light source and it allows your doctor to view images on a video monitor. Colon cancer is the third leading cause of death in the US. It’s also preventable thanks to colonoscopies.