Which imaging feature distinguishes a simple renal cyst from a malignant lesion?

• A. Irregular thick wall with nodules on ultrasound
• B. Echoic round mass with smooth and sharply demarcated wall on ultrasound
• C. Enhancement of a lesion on CT after contrast
• D. Thickened wall with septations on CT

Answer: (B)

The key idea is that a simple renal cyst is a fluid-filled, non-solid structure with a clean, smooth border. On ultrasound this appears round and well defined with a thin, smooth wall and content that is fluid (anechoic), often showing posterior acoustic enhancement. That clean, sharply demarcated border is what most clearly distinguishes it from a malignant lesion, which tends to be more irregular, may have thickened walls, and can contain internal echoes or nodules.

Other signs of malignancy include an irregular or thickened wall with nodules, or internal septations and enhancement after contrast on CT, all of which indicate solid tissue or vascularized components rather than a simple cyst. So the smooth, sharply defined wall on ultrasound best fits a simple cyst pattern.