ACVIM Small Animal Internal Medicine (SAIM) Seminal Paper Practice Exam

Differentiation hinges on the cellular pattern of the infiltrate and how the mucosal architecture is affected. Inflammatory bowel disease in cats typically shows a polymorphic lymphoplasmacytic infiltrate with both T- and B-cell markers present, and relatively few intraepithelial lymphocytes (low intraepithelial counts). Small cell lymphoma, on the other hand, is a dense, monomorphic population of small T cells (CD3+), with a high intraepithelial lymphocyte count (many IELs, >40 per 100 enterocytes) and marked villous blunting or fusion due to lymphoid infiltration. So the pattern described as a mixed lymphoplasmacytic infiltrate with mixed T/B cell markers and sparse intraepithelial lymphocytes corresponds to IBD, while a heavy monomorphous CD3+ infiltrate with many intraepithelial lymphocytes and villous atrophy is characteristic of small cell lymphoma.