Peripheral lymphadenopathy

A 51-year-old man with unremarkable past medical history was found to have, on the occasion of a routine medical examination, a white blood cell count of 18,000 per microliter. The hemoglobin level was 14.2 g/dL, and the platelet count was 210,000 per microliter. The physical examination was normal with no peripheral lymphadenopathy, spleen, or liver enlargement being noted. The differential white blood cell count revealed 80% mature-looking lymphocytes. The bone marrow examination revealed 75% lymphocytes and a non-diffuse histopathological pattern. Biochemical parameters, including serum lactate dehydrogenase and β2-microglobulin, were within normal values. Discuss your diagnosis.