In abdominal ultrasound, which feature distinguishes a simple cyst from a solid mass?

• A. Simple cyst is hyperechoic with posterior acoustic shadowing; solid mass is anechoic.
• B. Simple cyst is anechoic with posterior acoustic enhancement; solid mass is hypoechoic or complex with internal echoes.
• C. Simple cyst has internal echoes and irregular walls; solid mass is completely anechoic.
• D. Simple cyst shows no posterior enhancement; solid mass shows posterior enhancement.

Answer: (B)

Fluid-filled structures behave differently on ultrasound than solid tissues. A simple cyst is anechoic, meaning it looks completely dark inside because there are no internal reflections from fluid. It also shows posterior acoustic enhancement—the bright area behind the cyst—since sound passes through fluid with little attenuation. In contrast, a solid mass contains tissue that reflects sound, so it is not anechoic and often appears hypoechoic or heterogeneous with internal echoes, and it lacks the clean posterior enhancement seen with cysts (though it may show complex features or shadowing if calcifications are present). Therefore, the combination of an anechoic interior with posterior enhancement is the best sign of a simple cyst, while internal echoes or a complex/heterogeneous appearance points to a solid mass. The other statements contradict these acoustic physics principles (for example, a cyst is not hyperechoic with shadowing, nor does it have internal echoes or lack posterior enhancement).