The most common cause of renal scaring in a 3 year old child is -
Trauma
Tuberculosis
Vesicoureteral reflux induced pyelonephritis
Interstitial nephritis
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
Kidney