Differential Diagnosis

The patient's complex medical history and presentation require a thorough differential diagnosis. The following categories outline the potential diagnoses:

• Single Most Likely Diagnosis
  • Recurrence of Primary Disease (e.g., AL Amyloidosis or Plasma Cell Dyscrasia): The patient's history of dense atypical plasma cell infiltrate, focal amyloid protein, and previous autologous stem cell transplant suggests a possible recurrence of the primary disease, which could be contributing to the progressive CKD and increased proteinuria.
• Other Likely Diagnoses
  • Sjögren's Syndrome-Associated Interstitial Nephritis: The patient's diagnosis of Sjögren's syndrome and the presence of interstitial fibrosis on the kidney biopsy suggest a possible contribution of Sjögren's-associated interstitial nephritis to the CKD.
  • Chronic Tubulointerstitial Nephritis: The patient's long-standing CKD, presence of interstitial fibrosis, and negative immunofluorescence results could indicate chronic tubulointerstitial nephritis, potentially secondary to the underlying disease or other factors.
  • Membranous Nephropathy: Although the immunofluorescence results are negative, the presence of rare subepithelial deposits on the kidney biopsy could suggest membranous nephropathy, which may be secondary to the Sjögren's syndrome or other autoimmune processes.
• Do Not Miss Diagnoses
  • Vasculitis (e.g., ANCA-Associated Vasculitis): Although less likely, vasculitis could be a potential cause of the patient's progressive CKD and increased proteinuria. Missing this diagnosis could have severe consequences, and therefore, it should be considered and ruled out.
  • Infection-Related Nephropathy: The patient's immunosuppressed state due to the use of MMF and prednisone increases the risk of infection-related nephropathy, which could be caused by opportunistic pathogens.
• Rare Diagnoses
  • Light Chain Proximal Tubulopathy: This rare condition is associated with plasma cell dyscrasias and could contribute to the patient's CKD.
  • Fibrillary Glomerulonephritis: Although the immunofluorescence results are negative, the presence of rare subepithelial deposits could suggest fibrillary glomerulonephritis, a rare condition that requires specific diagnostic testing.

Treatment Plans

The treatment plan will depend on the specific diagnosis and the patient's overall clinical condition. However, considering the differential diagnoses, the following general approaches could be considered:

• For recurrence of primary disease or plasma cell dyscrasia, treatment may involve re-initiation of chemotherapy, immunomodulatory therapy, or other targeted therapies.
• For Sjögren's syndrome-associated interstitial nephritis, treatment may involve immunosuppressive therapy, such as MMF or rituximab, and supportive care.
• For chronic tubulointerstitial nephritis, treatment may focus on supportive care, including blood pressure management, proteinuria reduction, and avoidance of nephrotoxic agents.
• For membranous nephropathy, treatment may involve immunosuppressive therapy, such as MMF or rituximab, and supportive care.

It is essential to note that the patient's intolerable side effects to prednisone and limited dose escalation of MMF due to leucopenia will require careful consideration when selecting a treatment plan. A multidisciplinary approach, involving nephrology, rheumatology, and hematology, may be necessary to optimize the patient's care.