Which of the following is a benign kidney mass?

• A. Angiomyolipoma
• B. Renal Cell Carcinoma
• C. Nephroblastoma
• D. Oncocytoma

Answer: (A)

The key idea is distinguishing lesions that are typically benign from those that are malignant. Angiomyolipoma is a benign kidney mass composed of abnormal blood vessels, smooth muscle, and fat. The presence of macroscopic fat on imaging is a classic clue that helps differentiate it from malignant tumors, and while it can grow large and bleed, it does not metastasize. That combination makes angiomyolipoma the best-known example of a benign renal mass.

Renal cell carcinoma is malignant, so it isn’t the correct choice for a benign mass. Nephroblastoma (Wilms tumor) is also a malignant tumor, typically in children. Oncocytoma is another benign lesion, but it’s less distinctive on imaging because it lacks the definite fat signal that angiomyolipoma shows and it can be mistaken for renal cell carcinoma, making it a trickier, less clear-cut benign diagnosis in many exams.