A 55-year-old man felt a lump near his shoulder 1 week ago. On physical examination, there is an enlarged, nontender, supraclavicular lymph node and enlargement of the Waldeyer ring of oropharyngeal lymphoid tissue. There is no hepato- splenomegaly. CBC is normal except for findings of mild anemia. A lymph node biopsy specimen shows replacement by a monomorphous population of lymphoid cells that are twice the size of normal lymphocytes, with enlarged nuclei and prominent nucleoli. Immunohistochemical staining and flow cytometry of the node indicates that most lymphoid cells are CD19+, CD10+, CD3-, CD15-, and terminal deoxynucleotidyl transferase negative (TdT-). A BCL6 gene mutation is present. Which of the following is the most likely diagnosis?