Simple Renal Cyst: Given the description of the lesion as hypodense and nonspecific with an attenuation of 25 HU, it is most consistent with a simple renal cyst. Simple cysts are common, benign, and typically appear as well-defined, hypodense lesions on CT scans due to their fluid content.

Renal Cell Carcinoma (RCC): Although less likely than a simple cyst given the hypodense nature, some RCCs can appear hypodense, especially if they have a high lipid content or are of the clear cell type with extensive necrosis. However, RCCs are typically more heterogeneous and may show enhancement with contrast.
Hemorrhagic Cyst: A cyst that has hemorrhaged can appear more complex and may have a higher attenuation value due to the presence of blood. However, the description provided does not strongly suggest recent hemorrhage.
Infectious or Inflammatory Lesion: Abscesses or inflammatory lesions can appear as hypodense masses, but they are usually associated with clinical symptoms of infection or inflammation and may show rim enhancement with contrast.

Renal Cell Carcinoma with Necrosis: As mentioned, while RCC is less likely given the description, it is crucial not to miss this diagnosis due to its significant implications for patient management and prognosis. Any solid component or enhancement within the lesion should prompt further investigation.
Angiomyolipoma (AML) with Minimal Fat: Although AMLs are typically characterized by their high fat content, some may have minimal fat and appear more hypodense. The presence of any fat within the lesion, even if minimal, is diagnostic of AML.

Lymphoma: Renal involvement by lymphoma can result in hypodense masses, but this is relatively rare and usually occurs in the context of known lymphoma.
Metastasis: Metastatic disease to the kidney can appear as hypodense lesions, but this is uncommon and typically seen in patients with a known primary malignancy.
Renal Lymphangioma: A rare benign lesion that can appear as a hypodense mass due to its fluid content.