Misleading leads: Wilms tumor with a metachronous lesion appearing in the remaining kidney

Medullary dysplasia may be present in the kidneys of children with the Beckwith-Wiedemann Syndrome (BWS). This is usually visible only at the microscopic level, but superimposed pyelonephritis in a child with the BWS led to gross changes that produced a filling defect on CT scanning of the remaining kidney. The finding could have been misinterpreted as a metachronous wilms tumor.