A 15-year-old patient complains of bleeding gums and pain during eating. These signs have been observed for the last year. Objectively the interdental papillae in the area of teeth 31, 32, 33, 41, 42, and 43 are enlarged and cover the tooth crowns by 1/2 of their height. The gums present with bright-red granulations and are painful on palpation. Mineralized dental plaque is observed on the teeth. X-ray shows no changes of the alveolar process. What is the most likely diagnosis?