A 57-year-old male presents with painless progressive enlargement of bilateral cervical lymph nodes for over 1 month, accompanied by intermittent irregular fever exceeding 38°C. Physical examination reveals one 3cm×2cm enlarged lymph node in each cervical region, and one 2cm×1cm enlarged lymph node in the left axilla and right groin, all mobile with no tenderness. Hodgkin lymphoma is clinically suspected. The best diagnostic method to confirm the diagnosis is: