For the same case, if a biopsy were performed on the surrounding bone marrow, what cell type would most likely be found (consistent with an underlying plasma cell disorder, e.g., multiple myeloma)?

Multiple myeloma is a malignancy of plasma cells, so a bone marrow biopsy in a patient with this underlying condition would show an abnormal, often sheet-like proliferation of plasma cells — distinguishing it from the other inflammatory cell types listed, which are not characteristic of a plasma cell neoplasm.