Hematologic Malignancies USMLE Step 1 Practice Question

A 58-year-old woman presents with a rapidly enlarging painless cervical lymph node and drenching night sweats for 3 weeks. Vital signs: BP 128/82, HR 98, RR 18, Temp 38.2°C, SpO2 98% on room air. Physical examination reveals no hepatosplenomegaly. Excisional biopsy demonstrates sheets of large atypical B cells that efface nodal architecture with CD20 positivity. CBC shows mild anemia (Hgb 10.2 g/dL) without eosinophilia. Which diagnosis is most likely?