Solution: Arteriovascular malformations can be congenital or acquired. If acquired, they usually occur following renal biopsy, trauma, or rupture of an aneurysm. It is actually difficult on the CT to determine if this is a AVM, AVF, or renal artery aneurysm. However, it is critical to note that it follows the vasculature on every phase and is certain to be a vascular structure. Therefore, you wouldn\'t want to do a biopsy. Fortunately, if you did an US, you would see a \"cystic\" structure and when you put on doppler, you would see extensive flow. However, the rim calcification might interfere with that assessment. Most common symptom is hematuria, although if it is high flow, you can have high flow CHF.