Sometimes, when doctors are not sure what's wrong with a part of your body, they might decide to do a test called a biopsy. This means they remove a tiny piece and look at it under a microscope. A common kind of biopsy is of the skin, but a doctor also can take a biopsy almost anywhere, including from a muscle, lung, or kidney. The biopsy gives doctors a closer look at what's going on inside and will help make the diagnosis so the doctor can choose the right treatment. A doctor will give you medicine called anesthesia that will keep you comfortable while the biopsy is done.