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 ane- mia. 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 promi- nent nucleoli. Immunohistochemical staining and flow cytom- try of the node indicates that most lymphoid cells are CD19+, CD10+, CD3,CD15-, and terminal deoxynucleotidyl transfer- ase negative(TdT-). A BCL6 gene mutation is present. Which of the following is the most likely diagnosis?