Xanthogranulomatous pyelonephritis: The conundrum of a calcified kidney

Xanthogranulomatous pyelonephritis (XGP) is an uncommon chronic destructive granulomatous inflammation of the kidney that can sometimes mimic other renal pathologies like tumours or infections. For this reason, it is also called "the great imitator".This is the case of a 7-year old girl that presented to the hospital with right hip pain and a suspicious right sided abdominal lesion, thought initially to be a Wilms' tumour and then an appendiceal mass with a psoas abscess. The abscess was percutaneously drained and the patient was treated with intravenous antibiotics. At 6 weeks interval, the interval appendicectomy was negative. On subsequent follow-up imaging, an abnormally structured kidney was shown, surrounded by a calcified rim and a remnant psoas abscess, which suggested XGP or tuberculous (TB) pyelonephritis as differentials. A DMSA scan showed no function in the affected kidney and the patient underwent a nephroureterectomy. The histopathology showed features that prompted consideration of tuberculosis but ultimately allowed a diagnosis of XGP.XGP can be a confounding pathology as in the present case, where there remained multiple clinical, radiological and histological differentials throughout the course of the patients care, right up to the final diagnosis.