BK Polyomavirus-Associated Urothelial Carcinoma Presenting as Obstructive Uropathy in a Long-Term Lung Transplant Recipient.

A man in his 40s on long-term immunosuppression with bilateral lung transplantation due to cystic fibrosis presented to the hospital with dysuria and flank pain, initially attributed to pyelonephritis. Repeated computed tomography revealed progressive bilateral hydronephrosis unresponsive to ureteric stenting, an extensive bladder wall lesion with calcification, and retroperitoneal lymphadenopathy. A cystoscopic biopsy reviewed at a tertiary centre confirmed high-grade urothelial carcinoma with extensive glandular differentiation, positive for the SV40 antigen, consistent with BK polyomavirus-associated carcinoma (G3 T1 N3 M1a). Persistent hydronephrosis despite appropriately situated stents was the pivotal imaging finding, prompting reassessment for extrinsic malignant obstruction and bilateral nephrostomy insertion. This case highlights the diagnostic challenge of distinguishing malignancy from infection in immunosuppressed individuals through serial imaging.