A 5-year-old girl complains of pain in the mouth angles occurring during mouth opening. She has a history of acute bronchitis and prolonged antibiotic therapy. Objectively: the red border is dry, the mouth angles are downturned. In the mouth angles there are cracks covered with white plaque and surrounded by slightly hyperemic elevation, painful on palpation and mouth opening. Oral mucosa exhibits no pathological changes, decay-missing-filled index - 2, hygiene index - 2,0. What is the most likely diagnosis?