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This renal biopsy shows extramedullary hematopoiesis (EMH), which is the presence of hematopoietic elements (erythroid, myeloid, and/or megakaryocytic) found outside of the bone marrow. The larger circle contains mostly erythroid precursors and the arrow identifies an immature megakaryocyte. Generally, the histologic differential diagnosis includes acute interstitial nephritis (erythroid precursors may be confused with mature inactive lymphocytes) and even some types of lymphoma (megakaryocytes may resemble atypical or malignant lymphoid cells). In difficult cases, lineage-specific immunohistochemical stains can be used to confirm the presence of hematopoietic cells. Recognizing EMH is important because it often signifies impaired bone marrow function due to...