Question

A 52-year-old man comes to the clinic for a routine evaluation. He reports feeling generally well but noticed a painless lump in his left axilla $2$ months ago that has not resolved. He has no fever, night sweats, or unintentional weight loss. Physical examination reveals a $3.5$-cm, firm, mobile, non-tender lymph node in the left axilla. The remainder of the examination is unremarkable. An excisional biopsy of the lymph node is performed. The pathology report describes a complete effacement of the normal lymph node architecture by a closely packed, nodular proliferation of cells. High-power microscopic examination shows predominantly small lymphocytes with irregular, cleaved nuclei admixed with a minor population of larger cells with prominent nucleoli. Immunohistochemical staining and flow cytometry demonstrate that the neoplastic cells are positive for CD19, CD20, CD10, and BCL-2, but negative for CD5 and CD23. Based on the histopathologic and immunophenotypic findings, which of the following is the most likely diagnosis?