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What is the Diagnosis? A 51-year-old woman with no significant medical history presented with a 4-week history of confluent lower limb erythema. Examination showed a gradually resolving rash. Laboratory tests revealed anemia (hemoglobin 64.0 g/L), hypoalbuminemia (29.8 g/L), transient renal impairment (peak creatinine 164 mmol/L), and proteinuria (1.75 g/d) without hematuria. ANCA testing was negative. Abdominal ultrasound showed normal renal size and echogenicity. Renal biopsy raised suspicion of malignancy; however, bone marrow biopsy was unremarkable. PET/CT showed no systemic abnormalities. Skin biopsy revealed limited intravascular CD20-positive lymphocyte aggregates and IGK gene rearrangement, con fi rming intravascular large B-cell lymphoma (IVLBCL). The patient declined treatment and died of hemophagocytic syndrome 5 months later. Image Description Kidney biopsy revealed atypical lymphocytes occupying a single Bowman space, mimicking crescent formation. Lymphoma cells were enlarged, approximately 3–4 times the size of normal lymphocytes. Immunohistochemistry was positive for CD20 and MUM-1 and negative for WT-1. Teaching Points Although hematogenous metastases from other malignant tumors, such as malignant melanoma, have been reported to mimic crescent formation, 1 renal IVLBCL with extravascular infiltration remains rarely described. IVLBCL, typically confined to the lumina of small-to-medium vessels likely due to deficiencies in adhesion and homing receptors, 2,3 often involves infiltration of glomerular and peritubular capillaries in the kidney. 4 We hypothesize that lymphoma cells may traverse disrupted or remodeled areas of the glomerular basement membrane, particularly at mesangial attachments, leading to pseudo-crescent formation. 1,5 This case expands the histopathologic spectrum of renal IVLBCL, enhancing the clinical and academic understanding of this rare condition. References 1.Ozluk Y, Kilicaslan I, Kazancioglu R, Demirkesen C, Derin D, Mandel NM. Intraglomerular crescentic metastases of malignant melanoma. Hum Pathol. 2011;42(12):2025–2028. doi:10.1016/j. humpath.2010.05.032 2.Sukswai N, Lyapichev K, Khoury JD, Medeiros LJ. Diffuse large B-cell lymphoma variants: an update. Pathology. 2020;52(1):53–67. doi:10.1016/j.pathol.2019.08.013 3.Schrader AMR, Jansen PM, Willemze R, et al. High prevalence of MYD88 and CD79B mutations in intravascular large B-cell lym-phoma. Blood. 2018;131(18):2086–2089. doi:10.1182/blood-2017-12-822817 4.Rastogi P, Alrwashdeh A, Caza T, et al. Characteristics of renal intravascular large B-cell lymphoma. Kidney Int Rep. 2023;8(3): 575–583. doi:10.1016/j.ekir.2022.12.001 5.Collar JE, Ladva S, Cairns TD, Cattell V. Red cell traverse through thin glomerular basement membranes. Kidney Int. 2001;59(6): 2069–2072. doi:10.1046/j.1523-1755.2001.00721.x (Text and image provided by Weiyuan Wang, Department of Pathology, Xiangya Hospital, Central South University, Changsha, China; Jiaxi Pu, Department of Nephrology, Xiangya Hospital, Central South University, Changsha, China; Yuhao Tao, Xiangya Stomatological Hospital and Xiangya School of Stomatology, Central South University, Changsha, China; Xiaomei Gao, Department of Pathology, Xiangya Hospital, Central South University, Changsha, China; Yuehua He, Department of Nephrology, Xiangya Hospital, Central South University, Changsha, China; Lijian Tao, Department of Nephrology, Xiangya Hospital, Central South University, Changsha, China; and Zhangzhe Peng, Department of Nephrology, Xiangya Hospital, Central South University, Changsha, China. W.W. and J.P. contributed equally to this work. Correspondence: Zhangzhe Peng) CJASN 21:1, 2026. DOI:10.2215/CJN.0000000776