What is the most common benign solid mass of the kidney and how does it appear on ultrasound?

• A. Renal adenoma: hyperechoic, heterogeneous, vascular.
• B. Renal oncocytoma: isoechoic with vascularity.
• C. Angiomyolipoma: hyperechoic, homogeneous, focal, non-vascular.
• D. Simple cyst: round, anechoic, with posterior enhancement.

Answer: (C)

Angiomyolipoma is the most common benign solid mass of the kidney. Its fat component makes it stand out on ultrasound as a bright, highly echogenic lesion. On gray-scale imaging it is typically well defined and homogeneous, appearing as a focal area rather than replacing the whole kidney. Color Doppler often shows little or no internal vascularity, which helps distinguish it from more vascular renal tumors. In comparison, a simple cyst would be round and anechoic with posterior enhancement and no solid tissue, while adenomas and oncocytomas have different echotexture and vascular patterns that are not as consistently bright and homogeneous.