A 26-year-old woman has painless enlargement of several cervical lymph nodes, night sweats and generalised itch. She has no recent dental infection, cat exposure or travel history. Computed tomography shows enlarged cervical and mediastinal nodes, and a node biopsy late in the assessment demonstrates large atypical cells with two mirror-image nuclei and prominent nucleoli in a mixed inflammatory background. Which one of the following is the most likely diagnosis?