Primary Wilms tumor of the urinary bladder.

Wilms tumor (WT) can occur at various extrarenal sites; however, the urinary bladder as the primary site is occasional. A 4-year-old-female child presented with difficulty in micturition for the past month. The contrast-enhanced magnetic resonance imaging with magnetic resonance (MR) urography revealed a polypoidal, heterogeneous mass in the urinary bladder with no abnormality in the kidneys. Cystoscopy-guided biopsy was reported as an extrarenal Wilms tumor (ERWT) with triphasic components. Post-chemotherapy, a computed tomography scan revealed a residual tumor for which she underwent partial cystectomy. The diagnosis of ERWT was confirmed. She received adjuvant chemotherapy and remained well at the 9th month post completion of chemotherapy. The primary bladder WT must be considered in the differential of a small blue round cell tumor at an extrarenal site in the pediatric age group. The diagnosis is especially challenging in small biopsy material, although it has immense significance in management and prognosis.