Which one of the following is the most common cause of abdominal mass in neonates –
"Multicystic dysplastic kidney is the most common cause of an abdominal mass in the newborn"  - Nelson

A most common abdominal mass is a neonate → Multicystic renal dysplasia.
A Most common abdominal mass in 1 month to 3 years → Wilms a tumour or multicystic renal dysplasia.
A most common renal problem in 3 to 6 years → Minimal change disease-causing nephrotic syndrome.
A most common renal problem in 6 to 14 years → Acute post-streptococcal GN.
Neuroblastoma
Wilm's tumour
Distended bladder
Multicystic dysplastic kidneys