Bedsores (also called pressure ulcers) are commonly referred to as being in stages. According to the United States National Library of Medicine, Stage I is the mildest stage of bedsore involving skin redness and pain, Stage II involves blisters or open sores, Stage III involves “craters,” or open, sunken holes in which tissue below the skin is damaged, and Stage IV bedsores are so deep that muscle and bone have been damaged.