Which histopathologic pattern differentiates inflammatory bowel disease from small cell lymphoma in cats?

• A. IBD: lymphoplasmacytic infiltrate, mixed CD3/CD21-CD79a positivity, sparse intraepithelial lymphocytes (0-20 per 100 villous enterocytes); LSA: heavy monomorphous CD3+ infiltrate, >40 intraepithelial per 100 enterocytes, marked villous blunting/fusion
• B. IBD: heavy monomorphous CD3+ infiltrate; LSA: lymphoplasmacytic infiltrate with CD3/CD21-CD79a positivity
• C. IBD: granulomatous infiltrate; LSA: neutrophilic infiltration
• D. IBD: CD20+ B-cell dominance; LSA: CD3+ T-cell exclusively

Answer: (A)

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.