A 21-year-old dental patient has been found to have a circular ulcer on the tongue dorsum. The ulcer is up to 1 cm in diameter, has demarcated raised edges and a deep inflammatory infiltrate at the base. The ulcer floor is flat, clean, of crimson color. The regional lymph nodes are of cartilaginous density, mobile, painless. The ulcer appeared a week ago and doesn't cause any discomfort. What kind of ulcer should be suspected in the first place?