The most common cause of renal scaring in a 3 year old child is -
Chronic pyelonephritis is characterized by renal inflammation and scarring induced by recurrent or persistent renal infection, vesicoureteral reflux, or other causes of urinary tract obstruction. VUR is a congenital condition that results from incompetence of the ureterovesical valve due to a sho intramural segment Ref Harrison20th edition pg 234
Trauma
Tuberculosis
Vesicoureteral reflux induced pyelonephritis
Interstitial nephritis
Kidney