A renal mass seen on prenatal checkup is most probably

Prenatal renal tumors:Enlargement of a kidney on prenatal imaging is usually due to hydronephrosis or a cystic renal enlargement, such as multicystic dysplastic kidney or polycystic diseaseSolid tumors of the kidneys are rarely seen in the antenatal period but can be diagnosed by ultrasound. Most develop properly as the baby is growing in the womb.(Refer: Nelson's Textbook of Pediatrics, SAE, 1st edition, pg no. 2467)