A 44-year-old male presents with fever, chills, and myalgia for 1 week, initially treated as a common cold but fever persisted and worsened. Over the past 2 days, he developed gingival bleeding and petechiae. Physical exam: temperature 40°C, clear lungs, spleen 2 cm below the costal margin, sternal tenderness. Lab tests: Hb 68g/L, WBC 46.3×10<sup>9</sup>/L, PLT 36×10<sup>9</sup>/L. Bone marrow examination confirmed acute leukemia. The most likely cause of fever in this patient is: