A 12-year-old boy complains of fever up to 38 °C, weakness, headache, pain in the mouth, presence of vesicles and ulcers. The acute condition developed three days ago. The patient has a history of recent pneumonia treated with antibiotics. Objectively: oral mucosa is hyperemic and edematous. The mucosa of lips, tongue and cheeks has large erosions covered with fibrinous pellicle. The lips are covered with thick brown crusts. The back of the hand has papules of doublecontour colour. Which of the listed agents should be primarily used in the topical treatment?