Hydronephrosis is diagnosed when the renal collecting system dilatation exceeds which measurement?

• A. 10 mm
• B. 5 mm
• C. 15 mm
• D. 20 mm

Answer: (A)

Dilation of the renal collecting system is the key sign being tested. On ultrasound, the anteroposterior diameter of the renal pelvis is measured; a pelvis that is expanded beyond about 10 mm (1 cm) indicates hydronephrosis. This threshold reflects that the collecting system has enlarged significantly beyond normal, often due to urine outflow obstruction or reflux, causing backpressure into the pelvis and calyces.

Keep in mind that smaller dilations can be normal variants or transient, influenced by factors like bladder fullness or patient hydration, so the finding of a pelvis ≥10 mm is what solidifies a hydronephrosis diagnosis. If hydronephrosis is suspected, correlate with symptoms and consider repeat imaging or further studies if needed.