The Hidden Hitchhiker: Retrocaval Ureter Masquerading as Isolated Pelviureteric Junction Obstruction.

The coexistence of true pelviureteric junction obstruction (PUJO) and a retrocaval ureter in a single, non-duplicated collecting system is exceptionally rare, and this case represents a "hidden retrocaval ureter," in which an upstream PUJO masked the classical radiological features, transforming the diagnosis into an unexpected intraoperative discovery. Our case highlights such a peculiar case, where a 70-year-old man with longstanding intermittent right loin pain was found to have right hydronephrosis and a serum creatinine of 1.7 mg/dL. Computed tomography showed a markedly dilated extra-renal pelvis resembling PUJO. The radiological report suggested a ureteric stricture at that level. Magnetic resonance urography also showed an abrupt medial narrowing at the L3-L4 level, but the ureter could not be seen passing behind the inferior vena cava (IVC). The abrupt medial kink raised suspicion of a retrocaval ureter. During robotic surgery, the ureter was found coursing behind the IVC, confirming a retrocaval ureter. Surprisingly, dividing this segment did not decompress the tense renal pelvis. Further dissection revealed a separate narrowing at the true PUJ, and urine was released only after the pelvis was opened above this obstruction. The PUJO had prevented urine from entering the retrocaval segment, which explained why it was not visible on preoperative imaging. Both narrowed segments were excised, the ureter was brought anterior to the IVC, and pyeloureteral reconstruction was performed over a double-J stent. This case is unusual not only because the two conditions occurred together but also because one obstruction concealed the other.