Nontranslocation follicular lymphoma mimicking infectious mononucleosis: a diagnostic challenge in a 39-year-old male.

Follicular lymphoma is an indolent B-cell non-Hodgkin lymphoma typically characterized by the t(14;18)(q32;q21) translocation resulting in B-cell lymphoma 2 (BCL2)/immunoglobulin heavy chain (IGH) fusion. Nontranslocation variants are rare and may present with atypical clinical features that complicate diagnosis. The patient's presentation initially resembled infectious mononucleosis due to a false-positive monospot test; however, subsequent workup revealed follicular lymphoma lacking the canonical BCL2/IGH translocation. Extensive infectious, autoimmune, and nutritional evaluation was unrevealing. Cervical lymph node biopsy demonstrated follicular lymphoma with uniform BCL2 expression but no detectable BCL2/IGH fusion by Fluorescence in Situ Hybridization (FISH), confirming nontranslocation follicular lymphoma. The patient achieved complete remission after six cycles of obinutuzumab, cyclophosphamide, vincristine, and prednisone. This case illustrates a rare presentation of nontranslocation follicular lymphoma mimicking infectious mononucleosis and emphasizes the importance of reconsidering hematologic malignancy in patients with persistent mucocutaneous inflammation and incongruent laboratory findings.