A 24-year-old male developed icteric hepatitis B 3 months ago. For the past week, he has had fever and bleeding from skin and mucous membranes. Lab findings: hemoglobin 70 g/L, RBC 2.5×10¹²/L, WBC 1.5×10⁹/L (differential: neutrophils 0.2, lymphocytes 0.78, monocytes 0.02), reticulocytes 0.001. Bone marrow smear shows significant reduction in erythroid, myeloid, and megakaryocytic series, with lymphocytes 0.74. The most likely diagnosis is: