A 15-year-old male presents with chills, high fever for 3 days, accompanied by epistaxis and oral ulcers. Physical examination reveals scattered petechiae over the body, no enlargement of superficial lymph nodes, no sternal tenderness, and no hepatosplenomegaly. Blood tests show: Hb 100 g/L, WBC 1.0×10<sup>9</sup>/L (neutrophils 0.15, lymphocytes 0.85), PLT 12×10<sup>9</sup>/L, and reticulocytes 0.001. The most likely diagnosis is: