A 30-year-old male patient was admitted with persistent high fever, sore throat, and gingival bleeding for 1 week. Physical exam: temperature 40°C, pallor, marked pharyngeal congestion, mucosal ulceration, disseminated petechiae, no palpable lymph nodes, no hepatosplenomegaly, no sternal tenderness. Lab results: RBC 1.5×10¹²/L, Hb 46g/L, WBC 2.4×10⁹/L (N 20%, L 75%, M 5%), PLT 19×10⁹/L, reticulocytes 0.1%. The most likely diagnosis is: