 Aphthous ulcers typically begin as erythematous macules ( reddened , flat area of mucosa ) which develop into ulcers that are covered with a yellow @-@ grey fibrinous membrane that can be scraped away . An erythematous " halo " surrounds the ulcer . The size , number , location , healing time , and periodicity between episodes of ulcer formation are all dependent upon the subtype of aphthous stomatitis . 
