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