What is the most common cause of hydronephrosis?

• A. Urinary tract infection
• B. Kidney stone
• C. Vesicoureteral reflux
• D. Ureteropelvic junction obstruction

Answer: (B)

Hydronephrosis results from obstruction to urine flow, causing urine to back up and dilate the renal pelvis and calyces upstream of the blockage. The most common cause in adults is obstruction from a kidney stone traveling down the ureter and lodging somewhere along its path. When a stone blocks the ureter, pressure builds upstream, leading to dilation that can be seen on imaging and causes flank pain or hematuria as the stone passes or remains lodged. Other causes, like vesicoureteral reflux, UPJ obstruction, or infection alone, can contribute in specific populations (for example, reflux and UPJ are more relevant in children or congenital cases), but they are less common overall than stones as the obstructive culprit.