Vascular defects:
(1) ruptured aneurysm (abdominal aortic, renal, other)
(2) cystic medial necrosis in an artery
(3) segmental arterial mediolysis
(4) immune vasculitis
(5) infectious vasculitis
Tumors:
(1) renal (angiomyolipoma. renal cell carcinoma, transitional cell carcinoma, other)
(2) adrenal (myelolipoma, hemangioma, pheochromocytoma, other)
Infection:
(1) renal or adrenal tuberculosis
(2) retroperitoneal abscess
Renal disease:
(1) renal infarct
(2) renal calculi
(3) nephritis
(4) rupture of a renal cyst
Other causes:
(1) coagulopathy
(2) anticoagulation therapy
Differential diagnosis: traumatic retroperitoneal hemorrhage, which may be occult or show a delayed presentation
