Polyarteritis Nodosa (PAN): This is the most common form of medium vessel vasculitis, characterized by necrotizing inflammation of medium-sized or small arteries without glomerulonephritis or vasculitis in arterioles, capillaries, or venules. Justification: PAN is a well-documented cause of medium vessel vasculitis, often presenting with systemic symptoms such as fever, weight loss, and hypertension, along with organ-specific symptoms depending on the vessels involved.

Kawasaki Disease: An acute, febrile vasculitis syndrome that primarily affects children, involving medium-sized arteries, particularly the coronary arteries. Justification: Although more common in children, Kawasaki disease can occur in adults and is a significant consideration due to its potential for causing coronary artery aneurysms.
ANCA-Associated Vasculitis (specifically, Microscopic Polyangiitis and Granulomatosis with Polyangiitis): While these conditions more commonly affect small vessels, they can also involve medium-sized vessels and are important to consider due to their potential for renal and respiratory involvement. Justification: The presence of ANCA antibodies and the pattern of organ involvement can guide the diagnosis towards these conditions.

Infectious Vasculitis (e.g., due to bacteria, viruses, or fungi): Infections can cause vasculitis by direct invasion of the vessel wall or through immune complex deposition. Justification: Missing an infectious cause could lead to inadequate treatment and potentially fatal outcomes, emphasizing the need for thorough diagnostic workup including blood cultures and serological tests.
Vasculitis due to Connective Tissue Diseases (e.g., Lupus, Rheumatoid Arthritis): These conditions can cause vasculitis of various sizes of blood vessels, including medium vessels. Justification: The presence of a connective tissue disease can significantly impact the treatment approach and prognosis.

Primary Central Nervous System Vasculitis: A rare form of vasculitis that exclusively affects the vessels of the central nervous system. Justification: Although rare, this condition can present with medium vessel involvement and is crucial to diagnose due to its isolated CNS involvement and potential for severe neurological deficits.
Cogan Syndrome: A rare disorder characterized by vasculitis and inflammation of the eyes and ears, which can also involve medium-sized vessels. Justification: Its rarity and potential for significant morbidity if not recognized and treated promptly make it a diagnosis not to be overlooked in the appropriate clinical context.
Ergotism: A condition caused by excessive and prolonged use of ergotamine or ergot-derived medications, leading to vasospasm and vasculitis of medium and small arteries. Justification: Given its iatrogenic nature and the potential for severe vascular complications, ergotism is an important, albeit rare, diagnosis to consider in patients with a history of ergotamine use.