A 25-year-old patient complains of headache, an elevated body temperature, and a rash. According to the patient's medical history, three weeks ago he had a case of an acute respiratory viral infection. Objectively, the following is observed: petechial rash located symmetrically on the body, mainly on the legs and buttocks, body temperature 37.4°C, blood pressure 110/80 mm Hg. Complete blood count: erythrocytes 3.21012/L, hemoglobin 110 g/L, color index 0.9, platelets 100109/L, leukocytes 8.7*109/L, eosinophils 4%, basophils 0%, band neutrophils 7%, segmented neutrophils 56%, lymphocytes 26%, monocytes 7%, ESR 17 mm/hour. What is the most likely diagnosis in this case?