A 19-year-old male presents with dizziness, fatigue, and pallor for 1 year, along with gingival bleeding and petechiae for 1 month. Laboratory tests show Hb 60g/L, WBC 3.2×10<sup>9</sup>/L, PLT 30×10<sup>9</sup>/L, and bone marrow smear confirms aplastic anemia. What is the first-line treatment for this case?