A Rare Case of Intravascular Large B-cell Lymphoma Presenting as Fever of Unknown Origin With Unexplained Proteinuria.

Intravascular large B-cell lymphoma (IVLBCL) is a subtype of extranodal diffuse large B-cell lymphoma, characterized by neoplastic B-cell proliferation within blood vessels. Its nonspecific presentation, often as fever of unknown origin (FUO) and systemic symptoms, makes diagnosis challenging. We report the case of a 66-year-old male patient presenting with persistent fever, weight loss, and respiratory failure. Initial investigations, including bone marrow biopsy and imaging, were inconclusive. The lack of response to empirical therapy and the patient's worsening condition presented us with a diagnostic dilemma. The unexplained proteinuria led us to perform a renal biopsy, which confirmed IVLBCL through the identification of atypical B-cells in glomerular capillaries. This case highlights the importance of considering IVLBCL in persistent FUO cases and the role of specific tissue biopsy in diagnosis, even with nonspecific bone marrow studies. Early detection and prompt treatment are crucial, but prognosis remains poor due to diagnostic delays.