Answer:
This usually occurs when an obstruction in the ureter, tube leading from kidney pelvis to bladder, occurs, such as in case of a kidney stone lodged in the ureter. It can also happen if the bladder cannot empty properly, such as when the prostate is very enlarged or there is urinary retention for any reason, like nerve injury to bladder, trauma to urethra, the tube leading out of bladder. The distention usually corrects itself once the obstruction is relieved. Distention of kidney pelvis can lead to infection of kidney and pain in flank.