A 20-year-old male presents with fever for 2 weeks, accompanied by skin bleeding, tenderness at the lower sternum (+), and spleen palpable 1.5 cm below the costal margin. Lab results: hemoglobin 80 g/L, white blood cell (WBC) count 2.0×10<sup>9</sup>/L, platelet count (PLT) 35×10<sup>9</sup>/L. The most likely diagnosis is: