A 3-year-old boy is brought in by his mother after she was giving him a bath and noticed a mass in his abdomen. She reports no significant past medical history. The child is quiet on examination. There is a palpable abdominal mass in the left quadrant, which does not cross the midline. It is not pulsatile but firm. Tenderness is elicited on deep palpation. Urine shows a trace amount of blood. What is the most likely diagnosis?