A 32-year-old male factory worker presents with irregular fever for over 2 months, accompanied by decreased appetite, weight loss, and night sweats. Physical examination reveals mild icterus of the sclera, three peanut-sized hard cervical lymph nodes with no tenderness, and two bean-sized inguinal lymph nodes on the right side. The spleen is palpable 2 cm below the costal margin. Laboratory tests show Hb 79g/L, reticulocytes 8%, total bilirubin 50μmol/L (direct bilirubin 12μmol/L), urine urobilinogen 1:160 positive, and urine bilirubin negative. Malignant lymphoma is suspected. The most appropriate diagnostic method is: