How are reflux and pyelonephritis related?

Sometimes. Reflux is the abnormal passage of urine from the bladder to the kidneys through the ureters, or urinary drainage tubes from the kidneys to the bladder. This usually does not occur. Reflux itself does not lead to kidney infections, however, if the urine is infected, reflux may lead to pyelonephritis. This is most common in young, female children.
Bacteria reach kidne. When bacteria gain access to bladder during a UTI they can reach the kidney because ureteral valves in subjects with reflux can back up to kidney. Valves usually block urine from going backwards toward the kidney. Once bacteria reach kidney they multiply and can cause pyelonephritis.