Polar renal scarring in pyelonephritis?
It's more an issue of the extent of reflux than anything else. If you look at the data on scarring, you'll see that grade III and IV VUR present with lateral scarring as well. So, any pyelonephritis that is associated with gross dilation of the ureter and collecting system will give diffuse scarring. Mild dilation, you'll get scarring where the reverse flow goes first. I can't answer the question about why the reverse flow goes to the poles first. The authors of the paper I linked have their own speculation. Robbins has another. Neither have evidence on the mechanism.