If multiple bands are present on PARR in a dog with lymphadenopathy, what is the most likely diagnosis?

The presence of multiple bands on a PCR for antigen receptor rearrangements (PARR) indicates a polyclonal population of lymphocytes. This finding is commonly associated with inflammatory diseases rather than neoplastic processes. In the case of malignant lymphoma, a monoclonal band would be expected, signifying a single clone of neoplastic lymphocytes dominating the response.

Polyclonality reflects a response from various lymphocyte clones, which suggests that the lymphadenopathy is likely due to an inflammatory process rather than malignancy. Conditions such as reactive lymphadenopathy from infections or autoimmune diseases often produce a diverse array of lymphocyte populations, correlating with the observation of multiple bands on PARR.

Other potential options, like viral infections or chronic lymphocytic leukemia, may show different immunophenotypic or clinical characteristics, but they would not typically present as multiple distinct bands on PARR reflecting a polyclonal response. Thus, the observation of multiple bands strongly supports the diagnosis of an inflammatory disease.