Figure 4. Histological and immunohistochemical features of the second excised elbow lesion. Even at low-power overview, the histopathological picture of the second sample (A: H&E ×50) clearly overlaps with that of the previously excised nodule, showing the tumor-like deposit of glassy pink amorphous material admixed with and focally overgrowing a lymphoplasmacytic infiltrate composed of small lymphocytes, plasma cells, and plasmacytoid lymphocytes, with numerous Dutcher bodies (B: H&E ×200). By immunohistochemistry, numerous PAX5+ B cells (C: PAX5 IHC ×100) are found in association with CD138+ plasmacytic cells (D: CD138 IHC ×100), and both cellular populations demonstrate Ig κ light-chain restriction (E: Ig k IHC ×200). As in the previous biopsy, the positivity for Congo red stain (F: ×100) and related apple-green birefringence under polarized light (inset: ×200) confirms the material is indeed amyloid.