Correct: B

Histology: The lesion shows a relatively well circumscribed mass composed of a mixed inflammatory infiltrate with sheets of foamy histiocytes.

Discussion: Xanthogranulomatous pyelonephritis is a variant of chronic pyelonephritis in the setting of chronic obstruction due to renal calculi, which may lead to massive unilateral destruction of the kidney with the presence of granulomatous tissue that contains lipid-filled macrophages. Clinically, the patients can present with back pain, fever, or incidentally found in imaging studies performed for another reason. Image studies reveal an infiltrative mass that is suspicious for carcinoma. Three forms of this condition have been described: diffuse, segmental, and focal. The diffuse type is the most common one, and there are three stages associated with it. The first one is the nephric stage, which is confined to the renal parenchyma; stage II, or the perinephric stage, involves the perirenal space and Gerota’s fascia; and stage III, or the paranephric stages, refers to the involvement of the pararenal area and retroperitoneal structures. The differential diagnosis include other histiocytic disorders such as Rosai Dorfman disease, Erdheim Chester disease and malakoplakia. Malakoplakia has similar histologic features but it is characterized by the presence of Michaelis Gutman bodies which are small targetoid concentrically layered basophilic inclusions. Histiocytes of Rosai Dorfman have eosinophilic cytoplasm and are positive for S100 by immunohistochemistry. Erdheim Chester disease is an extremely rare neoplastic process that could be diagnosed using immunohistochemistry to detect mutant BRAF V600E.

References:
1. Cureus. 2021 Oct; 13(10): e19133.