What is the most likely diagnosis for a patient with inflammation of medium-sized arteries and a negative ANCA test?

The presence of inflammation in medium-sized arteries along with a negative ANCA (antineutrophil cytoplasmic antibodies) test strongly suggests polyarteritis nodosa (PAN) as the most likely diagnosis. PAN is a type of systemic vasculitis that primarily affects medium-sized muscular arteries, leading to ischemia and inflammation in various organs without the renal or respiratory involvement typically seen in ANCA-associated vasculitides.

In this condition, the absence of ANCA is a key differentiating factor, as eosinophilic granulomatosis with polyangiitis and granulomatosis with polyangiitis are both associated with positive ANCA tests. Henoch-Schönlein purpura primarily affects small vessels and is related to IgA deposition, making it distinct from PAN.

In summary, the characteristics of medium-sized artery involvement combined with the negative ANCA test clearly align with polyarteritis nodosa, making it the most appropriate diagnosis in this scenario.