Beneath Sepsis, Lupus-Like Autoimmunity, and COVID-19: Diagnostic Eclipse in Fatal Intravascular Large B-cell Lymphoma.

Intravascular large B-cell lymphoma (IVLBCL) is a rare extranodal neoplasm in which malignant B cells proliferate predominantly within small blood vessels. Because lymphadenopathy, mass lesions, circulating malignant cells, and specific radiological findings may be absent, IVLBCL can closely mimic other medical conditions, resulting in delayed diagnosis. This case is notable for a prolonged culture-negative sepsis-like presentation with persistent unexplained hypoxemia due to predominant pulmonary microvascular involvement, further obscured by lupus-like autoimmune features and subsequent COVID-19 infection. An older woman in her early 70s presented with fever, lethargy, weight loss, back pain, hypotension, raised inflammatory markers, cytopenia, markedly elevated lactate dehydrogenase (LDH), and progressive hypoxemia. She was initially treated for presumed sepsis, but repeated microbiological investigations and serial imaging did not identify an infectious source, and clinical improvement was not sustained. A transient malar rash and a positive antinuclear antibody raised concern for a lupus-like autoimmune disease; however, testing for anti-double-stranded DNA and extractable nuclear antigen was negative, and the broader autoimmune workup did not support systemic lupus erythematosus as the unifying diagnosis. Subsequent severe acute respiratory syndrome coronavirus 2 infection further complicated the interpretation of hypoxemia and deterioration. Despite antimicrobial therapy, corticosteroids, antiviral treatment, and supportive care, she developed progressive multiorgan failure and died approximately six weeks after admission. Postmortem examination revealed widespread multiorgan IVLBCL involving the lungs, heart, kidneys, liver, spleen, and multiple additional extranodal sites. Pulmonary capillary involvement provided a clinicopathological explanation for persistent unexplained hypoxemia. This case demonstrates how IVLBCL may remain concealed when several plausible diagnoses coexist. Persistent culture-negative fever, constitutional decline, cytopenia, high LDH, nondiagnostic imaging, treatment nonresponse, and unexplained hypoxemia should prompt consideration of IVLBCL and early tissue-based investigation.