Renal replacement lipomatosis of a single moiety in a horseshoe kidney without calculi or infection: A rare CT diagnosis.

Renal sinus lipomatosis is a benign proliferation of fatty tissue within the renal sinus, typically preserving the renal parenchyma, and is usually detected incidentally. Renal replacement lipomatosis (RRL) represents the advanced end of the spectrum of renal sinus lipomatosis. It is characterized by extensive fatty proliferation associated with marked renal parenchymal atrophy. Horseshoe kidney is the most common renal fusion anomaly; however, its association with unilateral RRL is rarely reported. We report a case of a 28-year-old male who presented with right flank pain and was found to have a horseshoe kidney with near-complete RRL of the right moiety on CT urography. He had no history of renal surgery, calculus disease, or infection. This case highlights the importance of recognizing RRL in a horseshoe kidney to avoid misdiagnosis and underscores the diagnostic value of CT urography in establishing the diagnosis.