A 38-year-old woman presents with a rapidly enlarging abdominal mass and B symptoms (fever, night sweats, weight loss) for 6 weeks. Imaging shows a large retroperitoneal lymph node mass. Fine-needle aspiration cytology shows large cells with high mitotic activity. Which investigation is most appropriate to confirm the diagnosis of diffuse large B-cell lymphoma (DLBCL) and assess for prognostic markers?