Abdominal Ultra-sonography in a 3 year old boy shows a solid well circumscribed hypo-echoic renal mass. Most likely diagnosis is:

Nephroblastoma (Wilm's tumour) is the commonest renal tumour in children and on USG appears a well circumscribed hypo echoic, solid mass. Ref: Differential Diagnosis in Abdominal Ultrasound By Bisset, 3rd Edition, 2008, Preview