Answer: Rheumatoid nodule

Histology: The lesion is present in the deep soft tissue and shows necrobiotic granulomas, which refers to areas of altered connective tissue in which there is blurring and loss of definition of the collagen bundles, a loss of connective tissue nuclei, and an alteration in staining with increased eosinophilia. The necrobiotic areas are partially or completely surrounded by a histiocytic rim, which contain multi-nucleated giant cells. In some foci, the histiocytes appear spindle-shaped and form palisades around the necrobiotic areas. The central necrobiotic focus is homogeneous and eosinophilic. Fibrin may be seen. In some areas, there is dense fibrosis with clefts and cystic degeneration.

Discussion: Granuloma annulare has very similar histologic features to rheumatoid nodules, with similar areas of necrobiosis and palisading histiocytes and lymphocytes. The most helpful feature is identifying mucin in the necrobiotic foci in granuloma annulare, which is not seen in rheumatoid nodules. The use of special stains such as a colloidal iron or an alcian blue can aid in the demonstration of the mucin present in the central necrobiotic areas. In addition, the clinical history is also helpful. Necrobiosis lipoidica was originally called necrobiosis lipoidica diabeticorum. Although many cases are associated with diabetes mellitus, not all patients with necrobiosis lipoidica have diabetes. The histology of the lesions vary. Biopsies will show a superficial and deep perivascular and interstitial mixed inflammatory cell infiltrate. A necrotizing vasculitis with adjacent areas of necrobiosis and necrosis of adnexal structures are also seen. The areas of necrobiosis may be extensive or scarce, and are often more extensive and less well defined than granuloma annulare. In contrast to granuloma annulare and rheumatoid nodules, necrobiosis lipoidica may show necrobiotic areas that are less frequent. Collections of epithelioid histiocytes and multi-nucleated cells, particularly around dermal vessels, are also seen in necrobiosis lipoidica. In addition, the necrobiosis seen in necrobiosis lipoidica is “open-ended,” in contrast to the necrobiosis seen in granuloma annulare and rheumatoid nodules, which show an inflammatory cell infiltrate that completely surrounds the area of necrobiosis. Small amounts of mucin may be seen in the necrobiotic areas of necrobiosis lipoidica; however, the presence of large amounts of mucin would favor a diagnosis of granuloma annulare.