List four causes of an AV fistula.

1. Penetrating or blunt trauma
2. Rupture of an aneurysm (e.g., aortocaval fistula following rupture of an abdominal aortic aneurysm into the IVC)
3. Iatrogenesis (e.g., created for dialysis access, surgical arterial injury, inadvertent simultaneous puncture of an artery and a vein during angiography)
4. Renal biopsy