Infection of the AV Fistula: This is a common complication in patients with vascular access for dialysis. The presence of a foreign body (the fistula) increases the risk of infection, which can lead to FUO.

Catheter-Related Bloodstream Infection (CRBSI): Although the patient has a native vein AV fistula, if there have been any temporary catheters used, CRBSI could be a source of FUO.
Dialysis-Related Infections: Infections related to the dialysis process itself, such as peritonitis in peritoneal dialysis (less likely here since the access is a native vein AV fistula, suggesting hemodialysis) or exit site infections.
Chronic Bacterial Infections: Such as endocarditis, especially in the context of a prosthetic valve or previous endocarditis, though less common without specific risk factors.

Endocarditis: Although it might not be the first consideration without specific cardiac risk factors, endocarditis can be deadly if missed. The presence of a vascular access increases the risk of bacteremia, which can lead to endocarditis.
Osteomyelitis or Septic Arthritis: These conditions can present subtly, especially in patients with chronic diseases, and are critical to identify early due to their potential for significant morbidity and mortality.
Intra-abdominal Infections: Such as diverticulitis, cholecystitis, or abscesses, which can be masked by the chronic illness and dialysis but are life-threatening if not addressed.

Tuberculosis: Although less common, TB can cause FUO, especially in immunocompromised patients or those with chronic diseases like CKD.
Fungal Infections: Such as histoplasmosis or cryptococcosis, which are rare but can occur in immunocompromised patients, including those on long-term dialysis.
Non-infectious Inflammatory Conditions: Like giant cell arteritis or polymyalgia rheumatica, which can present with FUO and are important to consider in the differential diagnosis of older adults.