[Unusual duplication of renal collecting system mimicking supernumerary kidney--a case report]

Unusual mode of renal duplication may be confused with supernumerary kidney. The presented adult female complaining left flank pain had bilateral duplication of collecting system with ectopic opening in one of the left-sided ureter. Since the ureteral opening to the bladder neck resulted in giant hydronephrosis in the upper half segment responsible for her complaint, surgical resection was performed. The duplication of the left renal mass and collecting system was regarded as "unusual" in several points. First, the lower half segment of the left kidney looked a normal complete kidney, because it had upper, middle and lower calices. Second, the lower half segment was located rather superior to the upper one. Third, the parenchyma of the upper segment which was a rudimentary small mass of several grams was separated from the lower one. But the left renal artery was single, which supplied blood to not only the lower segment but also the upper one, and the two segments were connected tightly each other by loose areolar tissue. These facts prevented to categorize this case as supernumerary kidney. This case suggests that there may be many transitional cases between fused kidney and supernumerary kidney.