A 20-year-old male presents with pallor and fatigue for 3 months. Physical examination reveals scattered petechiae on the skin, no enlarged superficial lymph nodes, no sternal tenderness, and no hepatosplenomegaly. Blood tests show: RBC 2.0×10¹²/L, Hb 60g/L, WBC 3.6×10⁹/L (neutrophils 30%, lymphocytes 70%), PLT 30×10⁹/L, and reticulocytes 0.004. The most likely diagnosis is: