What is vesicoureteral reflux (VUR, vesico-ureteric reflux)? Urine flows from the renal pelvis through the ureter into the bladder. When the bladder is full, the detrusor muscle contracts and expels the urine from the bladder through the urethra. In normal people, when the bladder contracts to urinate, urine does not return from the bladder into the ureter or kidney. However, 60-70% of infants who have urinary tract inflammation have vesicoureteral reflux (Baker, 1966), which means that during urination, some of the urine returns from the bladder into the ureter or even into the renal pelvis. Why does vesicoureteral reflux cause nephritis? It is because a significant proportion of bacteria (70-90% E. coli, E. coli) in urinary tract infections enter the bladder from the urethra. If the amount is small, it will be excreted in the urine with little or no symptoms. In large numbers, it can cause cystitis. The rate of urinary tract infections is 12 times lower in circumcised boys than in uncircumcised ones because the glans of the penis is exposed and relatively dry, the amount of bacteria between the foreskin and the glans is greatly reduced, and fewer bacteria enter the bladder via the urethra. The classic study by Ransley and Risdon 1979 found that reflux combined with bacterial infection could damage the kidneys, and when I went to London Children’s Hospital in 1995 for a refresher course in pediatric urology, Ransley, who was working there at the time, took a year off and was not able to attend. Urinary infections in children are very common and are the second most common source of infection after respiratory infections. Five percent of fevers in children between two months and two years of age are caused by urinary tract infections. In the age group of one to five years, the chances of urinary tract infections are greater in girls, 10-20 times more than in boys. However, for infants under three months of age, boys are 5-8 times more likely than girls, which is related to the high incidence of congenital urinary tract malformations in males. Why is there vesicoureteral reflux? The ureter in a normal child enters the bladder at an angle, meaning that the ureter weaves diagonally through the bladder wall for a distance (5 times the diameter of the urethra, Paquin 1959) before entering the bladder lumen. When the bladder is full, the pressure of the bladder urine flattens the ureter inside the bladder wall, closing the inner lumen and creating an anti-reflux mechanism. In those children with reflux, the ureter enters the bladder at an angle close to right angles and loses its anti-reflux mechanism. When the bladder fills and contracts to urinate, the pressure in the lumen not only expels the urine from the body but also causes vesicoureteral reflux at the same time.