Solution: The MRI shows a multi-lobulated cystic mass in the presacral space. It is well defined, shows no evidence of invasion into adjacent structures and has mixed signal with both T1 and T2 bright areas, but no real enhancement, except potentially some mild enhancement of the septations. These findings are consistent with a retrorectal cystic hamartoma, or "tailgut cyst". These cysts are thought to arise from remnants of the embryonic hindgut. They are 3 times more common in women and have rare malignant degeneration, but are generally benign. About half of patients are asymptomatic and about half have pain or mass effect symptoms. These are treated with surgical excision.