Simple Renal Cyst: This is the most common benign renal lesion, characterized by a well-defined, smooth margin and a hypodense (anechoic) center on ultrasound due to the fluid-filled nature of the cyst.

Renal Abscess: Although less common than simple cysts, renal abscesses can present with a hypodense center due to the fluid collection within the abscess. The smooth margin may be less typical, but it's possible, especially if the abscess is well-contained.
Hyalinized Hematoma: A hematoma that has undergone hyalinization can appear as a smooth, well-defined mass with a hypodense center due to the organization and liquefaction of blood products over time.

Renal Cell Carcinoma (RCC) with Central Necrosis: Although RCCs are typically solid and may have irregular margins, some can present with central necrosis, giving a hypodense appearance on ultrasound. Missing this diagnosis could be catastrophic, as RCC requires prompt treatment.
Metastasis to the Kidney: Metastatic lesions to the kidney can have a variable appearance, including a hypodense center if there is central necrosis. Given the potential for metastasis from various primary cancers, this diagnosis should not be overlooked.

Multilocular Cystic Nephroma: A rare, benign tumor that can have a complex appearance with multiple cystic spaces, potentially appearing as a hypodense mass on ultrasound if the cysts are large and dominant.
Lymphoma Involving the Kidney: Primary or secondary lymphoma of the kidney can present as a renal mass, potentially with a hypodense center if there is significant necrosis within the tumor. This is a rare diagnosis but should be considered in patients with known lymphoma or those with systemic symptoms.