Hydronephrosis is defined as distention of which renal structures with urine due to obstruction distal to the renal pelvis?

• A. Distention of renal calyces & pelvis with urine due to obstruction distal to the renal pelvis.
• B. Dilation of the ureter due to reflux.
• C. Inflammation of the kidney parenchyma.
• D. Enlargement of the renal pelvis without obstruction.

Answer: (A)

Hydronephrosis is dilation of the kidney’s collecting system due to urine backing up from an obstruction downstream. The collecting system consists of the renal calyces and the renal pelvis, so the defining finding is distention of both the calyces and pelvis with urine when there is an obstruction distal to the renal pelvis. This back-pressure mechanism explains why the calyces and pelvis enlarge rather than just the ureter or the kidney tissue itself. If the ureter alone were dilated from reflux, that would be hydroureter; inflammation of the kidney parenchyma describes nephritis; and dilation of the pelvis without any obstruction wouldn’t be hydronephrosis.