Pathology:

Left acetabular bone, Tru-cut biopsy: Plasmacytoma.

Note: The tumor cells are positive for CD138 and show lambda light chain restriction. Kappa light chain is negative by immune-histochemistry. CD56 shows weak focal positivity.

The free light chain pattern is compatible with polyclonal pictures, while serum protein electrophoresis showed the presence of 1.5 g/dL. M protein level is less than 3 g/dL. 

By the giving above data; features are representing stage I multiple myeloma.  Solitary plasmacytoma of the bone.