This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Which of the following blood vessels is affected in hypersensitivity vasculitis? 
Select and explain. 
A. Capillaries, B. Aerioles, C. Postcapillary venules, D. Medium sized vessels
C: Postcapillary venules
Answer: C: Postcapillary venules 
Explanation: Hypersensitivity vasculitis (HV) encompasses a heterogeneous group of vasculitides associated with hypersensitivity to antigens from infectious agents, drugs, or other exogenous or endogenous sources. It is characterized pathologically by involvement of postcapillary venules and inflammation and fibrinoid necrosis (necrotizing vasculitis). Clinically, skin involvement is characteristic, manifested by "palpable purpura." Systemic vascular involvement occurs, chiefly in the kidney, muscles, joints, GI tract, and peripheral nerves. Ref: Suurmond D. (2009). Section 14. The Skin in Immune, Autoimmune, and Rheumatic Disorders. In D. Suurmond (Ed), Fitzpatrick's Color Atlas & Synopsis of Clinical Dermatology, 6e.