The patient was a 79-year-old man who presented with 6 months of weight loss and decreased energy. Leading up to his presentation, he had lost 16 lbs (7.3 kg) over 6 months, despite having a good appetite. He had a past medical history of hypertension, asthma, and benign prostatic hypertrophy. There was no history of fevers, night sweats, rashes, and arthralgias. There was no known family history of kidney disease. His medications included simvastatin, mometasone, cetirizine, albuterol inhaler, and budesonide/formoterol. He was found to have an increase in his serum creatinine from a baseline of 1.1 mg/dL 2 years ago to 5.9 mg/dL during his most recent office visit. He was also found to have anemia, with a hemoglobin of 7.7 g/dL and new onset subnephrotic range proteinuria with a urine protein-to-creatinine ratio of 1.9 g/g creatinine. On his most recent laboratory studies, his serum albumin was 2.5 g/dL. He was nonoliguric, producing 1.0–1.5 L of urine daily. His routine serologic workup including hepatitis B, hepatitis C, HIV, ANA, ANCA, and anti-GBM antibodies were all unremarkable. His complement levels were normal. His urinalysis showed glucosuria (250 mg/dL). Urine microscopy showed 3–5 RBCs/HPF and 11–20 WBC/HPF. Overall, his serum and urine studies were concerning for a proximal tubule defect, given his hypokalemia (3.3 mmol/L), normal anion gap metabolic acidosis (serum bicarbonate of 21 mmol/L), glucosuria (250 mg/dL on urinalysis with a serum glucose of 105 mg/dL), and proteinuria. His serum protein electrophoresis revealed 2 restricted bands migrating in the gamma region. Serum free light chains showed elevated kappa light chains measured at 1,657 mg/L, lambda light chains at 22 mg/L, and a kappa/lambda ratio of 75. Kidney ultrasound showed that he had no hydronephrosis bilaterally, with his right and left kidneys measuring 11.1 cm and 11.9 cm, respectively. Therefore, a kidney biopsy was performed for further evaluation of the AKI and subnephrotic range proteinuria. Due to his age and abnormal light chain studies, there was a high suspicion for paraprotein-related kidney disease. The kidney biopsy demonstrated 42 glomeruli, 18 of which were globally sclerotic. The background tubulointerstitium showed moderate to severe interstitial fibrosis and tubular atrophy. The nonatrophic cortex showed interstitial inflammation, with a mixed inflammatory infiltrate comprising numerous reactive plasma cells, lymphocytes, monocytes, and scattered eosinophils (shown in Fig., top left). Scattered focal tubulitis and acute tubular injury were seen (minor component). In addition to this, some areas of the infiltrate had an atypical appearance composed of monomorphic lymphoid cells with hyperchromatic nuclei and scant cytoplasm. Glomeruli were relatively unremarkable, albeit for mild ischemic changes. Arteries and arterioles showed moderate intimal fibrosis and luminal narrowing. No atypical casts or crystal deposition was noted. Immunofluorescence (IF) stains revealed a unique pattern of tubulointerstitial immune complex deposition (shown in Fig., top right). Deposits were seen along proximal tubular basement membranes and along Bowman's capsule. Some glomeruli also showed segmental granular peripheral capillary wall staining. The glomerular and tubular basement membrane deposits showed IgG (2+), C3 (2+), kappa (2+), and lambda (1+) staining. Kappa showed a mild increase in background staining. C1q was negative. Patchy brush border staining was also seen with IgG. IgG subclass staining revealed a polytypic pattern to both glomerular and tubulointerstitial deposits (IgG1: 2+, IgG2: 0, IgG3: 0, and IgG4: 1+). Phospholipase A2 receptor was negative. Due to the unusual pattern of tubulointerstitial immune complex deposition, anti-LRP2 nephropathy was suspected, and therefore, LRP2 IF staining on formalin-fixed paraffin-embedded tissue was requested (EMD Millipore, Billerica, MA, USA: performed at Arkana Laboratories, Little Rock, AR, USA). The stains revealed positivity for LRP2 in the tubular basement membrane and Bowman's capsule deposits (shown in Fig. ). Glomerular deposits were negative for LRP2. Immunohistochemical (IHC) staining for SV40 (polyomavirus) was negative. IgG4 IHC staining revealed only rare IgG4-positive plasma cells. In retrospect, changes suggestive of glomerular and tubular basement membrane deposits were not readily visible by light microscopy. Electron microscopy confirmed the presence of electron-dense deposits along tubular basement membranes (shown in Fig., bottom left), Bowman's capsule, and segmentally in glomerular subepithelial locations (shown in Fig., bottom right). The deposits did not show substructure. No finely granular deposits or fibrillary or other organized forms of deposits were identified. Glomeruli were otherwise unremarkable, except for podocyte foot process effacement in areas of subepithelial immune complex deposition. The atypical lymphoid cells showed expression of CD20 (diffuse), PAX5 (diffuse), CD10 (subset), and BCL2 (shown in Fig. ). BCL6 was positive in a small subset of the neoplastic B-cells, and BCL1 was negative. CD43 was also expressed in a small subset of the neoplastic cells. CD5 and CD21 were negative. CD3 highlighted reactive T cells. The neoplastic cells also showed staining with kappa, while lambda was negative (in situ hybridization). Fluorescence in situ hybridization studies for BCL2, BCL6, and MALT1 gene rearrangements were negative. B-cell clonality studies on the tissue revealed clonal rearrangements in the heavy and kappa light chain immunoglobulin loci (IgH and IgK), consistent with B-cell clonality. Based on the morphological appearance and immunophenotypic findings, including positivity for CD43, BCL6, and aberrant CD10 expression in a subset of the cells, a diagnosis of lymphoplasmacytic lymphoma was considered less likely. MYD88 testing could not be performed due to lack of tissue. Therefore, the overall morphological, IHC, and molecular features were thought to be most consistent with a diagnosis of extranodal marginal zone lymphoma. Based on the clinical presentation and biopsy findings, the patient was diagnosed as having anti-LRP2 nephropathy. Subsequent radiological workup for systemic lymphadenopathy or masses was negative, and he was therefore diagnosed with concurrent primary extranodal marginal zone lymphoma of the kidney. He received 4 doses of rituximab at 375 mg/m2 per week for treatment of both his lymphoma and autoimmune disease. Twelve months after his initial diagnosis, he continues to remain dialysis-dependent. Follow-up on trends of hematological parameters was not available.