A 62-year-old male presents with dizziness and fatigue for 6 months, worsening over the past month, accompanied by low-grade fever and limb soreness. Physical examination: temperature 38°C, severe anemic appearance, petechiae on lower limbs, two peanut-sized palpable cervical lymph nodes, spleen 2 cm below the costal margin. Laboratory tests: Hb 50g/L, WBC 3×10<sup>9</sup>/L, PLT 45×10<sup>9</sup>/L, blood smear shows nucleated red blood cells (3/100 WBC), macrocytes, and giant platelets. The most critical diagnostic test is: