A 23-year-old man complains of gum bleeding when he brushes his teeth or eats solid food. Objectively: the gums of the front lower jaw are hyperemic, swollen and bleeding when palpated. Oral and gingival mucosa in other areas are not affected. The occlusion is deep. The teeth are firm, except for the 41 and 31 (degree 1 mobility). X-ray shows resorption of the alveolar septum in the area of the 41, 42, 32, and 31 teeth up to 1/3 of the root length. What is the most likely diagnosis?