Which of the following is NOT a criterion for a simple cyst?

In the context of evaluating the characteristics of a simple cyst, posterior shadowing is not a criterion for a simple cyst. A simple cyst typically appears as an anechoic (completely dark) area on ultrasound, indicating that it is filled with clear fluid.

The defining features of a simple cyst include smooth margins that are more distinct compared to solid masses, well-defined borders, and the presence of clear fluid within the cyst. These characteristics help differentiate simple cysts from other types of masses, which may exhibit irregular borders or content that isn't simply fluid.

Posterior shadowing, on the other hand, indicates that there may be a denser structure behind the cyst, which is not typically associated with simple cysts. In fact, simple cysts often do not produce posterior shadowing, as they do not contain solid materials or calcifications that would create this effect. This distinction helps in proper identification and characterization of lesions in diagnostic imaging.