the kidneys, their function in filtering blood, the measurement of glomerular filtration rate (GFR), and the abnormality of vesicoureteral reflux in children.

The kidneys are highly vascular organs that filter the blood, removing wastes for excretion and returning the rest of the blood to the vascular system. The functional units of the kidneys are nephrons, each composed of a glomerulus and a tubule. 
How is the GFR measured? What are the strengths and limitations of methods of determining GFR?
Often an incidental finding in children during a urinary tract infection workup, in vesicoureteral reflux (VUR), urine flows from the bladder back up the ureters.
Could you explain the mechanism of action of this abnormality in children?