This is a two views of the same section of a medium-sized artery. There is massive infiltration of the vessel wall characteristic of a vasculitis -specifically this has the appearance of polyarteritis nodosa. There was significant fibrin deposition on IF within the walls of the vessel. There was also some mesangial deposition of IgG and IgM.
This is a fascinating case. First, this form of vasculitis is not usually associated with a positive ANCA test and this may have been a red herring. Second, the smaller vessels were normal and if the arcuate artery was not present on the specimen, this patient would likely have been diagnosed with an interestitial nephritis. The proteinuria in this case is probably a result of reduced tubular reabsorption given the fact that there is no significant glomerular disease. The low serum albumin was most likely due to GI losses rather than renal.
Bonus History of Nephrology Point Although we associate interstitial nephritis with drug use and know that it was classically described in the setting of methicillin use, AIN was initially described in the setting of acute sepsis. Councilman nephritis was first described in 1898 in autopsy specimens of patients who died with sepsis. Given the plethora of drugs that most septic patients are exposed to these days prior to any biopsy, this is a difficult diagnosis to make at this point but it should be remembered that not all AIN is drugs. The image below is a plate from that paper which is available for free online.
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