American Board of Pathology (ABPath) Practice Test

A parapelvic cyst can indeed be challenging to differentiate from hydronephrosis due to its anatomical location and appearance on imaging studies. Parapelvic cysts are located in the renal sinus, adjacent to the renal pelvis and calyces, which can mimic the appearance of dilated renal collecting systems seen in hydronephrosis.

On ultrasound or CT imaging, both parapelvic cysts and hydronephrosis can present with similar characteristics such as contour changes in the renal outline or the presence of fluid collections that may extend into the renal pelvis. However, a parapelvic cyst typically does not communicate with the renal collecting system, which is a critical factor in distinguishing it from hydronephrosis.

Hydronephrosis indicates an obstruction in the urinary flow, leading to dilation of calyces and renal pelvis, while a parapelvic cyst is a benign lesion not associated with a functional obstruction. Therefore, careful evaluation using imaging modalities, along with clinical correlation, is essential in distinguishing between these two entities.