Which feature characterizes a simple renal cyst?

• A. Round, anechoic, thin walls, posterior enhancement.
• B. Irregular walls with internal echoes.
• C. Septated with calcifications.
• D. Solid, vascular mass.

Answer: (A)

Simple renal cysts are purely fluid-filled structures. On ultrasound they appear round or oval and completely anechoic, which means they look black because fluid does not reflect sound well. They have thin, smooth walls and show posterior acoustic enhancement, where the area behind the cyst appears brighter due to increased sound transmission through the fluid. Importantly, there are no internal echoes, no septations, no calcifications, and no solid components. Doppler typically shows no blood flow within the cyst. These features distinguish a simple renal cyst from complex cysts or solid renal masses, which would display irregular walls with internal echoes, septations or calcifications, or internal vascularity.