<prompt> Polyarteritis Nodosa PAN typically involves which of the following?Large elastic arteriesSmall- or medium-sized muscular arteriesArteriolesCapillaries  <response>: Ans b Small- or medium-sized muscular arteriesRef: Robbiris pathology 9th ed. 1506-507 PAN typically involves small- to medium sized muscular arteries. In contrast, large arteries and the aorta are involved in Takayasu arteritis. Small arteries and arterioles are involved in a number of other diseases, including systemic lupus erythematosus. Active lesions in PAN demonstrate a neutrophilic infiltration of the involved vessel wall with thrombosis and segmental,fibrinoid necrosis. Intermittent healing produces fibrosis of the- arterial wall and intimal thickening, which may lead to obstruction and infarction. Aneurysmal dilations may arise as a result of asymmetrical involvement. Although the lesions in PAN resemble other immune mediated vascular lesions, the exact etiology of the disorder has not been elucidated. PAN generally affects middle-aged men and has a poor prognosis, although steroids may be beneficial. 