The image shown comes from a kidney transplant patient with a rise in serum creatinine. What is the most likely diagnosis and what immunohistochemical stain would you want to rule out a less common disease sometimes seen in kidney transplants?

 

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The image shows lymphocytic tubulitis and surrounding lymphocytic inflammation. These findings are most commonly seen in acute T-cell mediated rejection (Banff IA or IB). However, BK/Polyoma virus nephritis can show similar findings and viral cytopathic atypia may not be present making an SV40 immunohistochemical stain useful in excluding this etiology.