What is the diagnosis for a 73-year-old female with cystic renal masses, including a Bosniak 4 left renal mass and Bosniak 3 right renal masses, accompanied by septic arthritis, piriformis myositis, and abscess, with laboratory results showing purulent fluid and abundant bacteria, but no malignancy?

Differential Diagnosis for 73F with Cystic Renal Masses and Septic Arthritis

• Single most likely diagnosis:
  • Xanthogranulomatous pyelonephritis (XGP) with associated abscesses and septic arthritis. This diagnosis is likely due to the presence of thick purulent fluid with abundant bacteria in the aspirate, chronic infection, and granulation tissue found in the intra-operative findings.
• Other Likely diagnoses:
  • Renal cell carcinoma with superimposed infection: The Bosniak 4 classification of the left renal mass and the presence of cystic masses in both kidneys raise the possibility of a malignant process, which could be complicated by an infection.
  • Pyogenic abscesses with septic arthritis: The patient's presentation with septic arthritis, osteomyelitis, and abscesses could be indicative of a pyogenic infection, potentially originating from the renal masses or as a separate entity.
  • Tuberculosis (TB) with renal and skeletal involvement: Although the stains for acid-fast bacilli (AFB) were negative, TB remains a possibility, especially given the chronic nature of the infection and the involvement of multiple systems.
• Do Not Miss diagnoses:
  • Metastatic disease with superimposed infection: It is crucial not to miss the possibility of metastatic disease, especially given the presence of multiple cystic masses and the patient's age.
  • Intra-abdominal or pelvic malignancy with abscesses and septic arthritis: Other malignancies, such as ovarian or gastrointestinal cancer, could present with similar symptoms and findings.
  • End-stage renal disease (ESRD) with infected cysts: The patient's renal masses and chronic infection could be indicative of ESRD with infected cysts, which would require prompt management.
• Rare diagnoses:
  • Malacoplakia: A rare inflammatory condition that can affect the kidneys and other organs, potentially presenting with similar findings.
  • Renal candidiasis with fungal balls and septic arthritis: Although the fungal stains were negative, this diagnosis remains a possibility, especially in immunocompromised patients.
  • Actinomycosis: A rare bacterial infection that can cause chronic abscesses and fistulae, potentially involving the kidneys and skeletal system.