Which protein is pathognomonic for multiple myeloma?

The presence of Bence Jones protein is indeed pathognomonic for multiple myeloma. This protein is a type of light chain immunoglobulin that is produced in excess by malignant plasma cells. In multiple myeloma, these abnormal plasma cells proliferate and produce monoclonal light chains, which can be found in the urine. When specific tests are conducted on urine samples, Bence Jones protein can be identified, and its presence supports the diagnosis of multiple myeloma.

Other options provided do not reflect the specific nature of multiple myeloma. For instance, smudge cells are typically associated with chronic lymphocytic leukemia (CLL), Auer rods are found in acute myeloid leukemia (AML), and Reed Sternberg cells are diagnostic of Hodgkin lymphoma. Thus, the distinct characteristic of Bence Jones protein in relation to multiple myeloma makes it the correct answer for this question.