**Core Concept**
Hypersensitivity vasculitis, also known as leukocytoclastic vasculitis, is a type of small vessel vasculitis characterized by inflammation and damage to the walls of small blood vessels. This condition typically affects the postcapillary venules, which are small blood vessels that allow blood to flow from capillaries to larger veins.

**Why the Correct Answer is Right**
The postcapillary venules are the primary sites of inflammation in hypersensitivity vasculitis. This is because these vessels are particularly susceptible to damage from immune complexes, which are abnormal proteins that form when antibodies bind to antigens. The immune complexes deposit in the walls of postcapillary venules, triggering an inflammatory response that leads to the characteristic symptoms of hypersensitivity vasculitis, such as palpable purpura, fever, and joint pain. The involvement of postcapillary venules is a key feature of this condition, distinguishing it from other types of vasculitis that affect larger or smaller vessels.

**Why Each Wrong Option is Incorrect**
**Option A:** Capillaries are the smallest blood vessels in the body, where the exchange of oxygen, nutrients, and waste products occurs. While capillaries can be affected in various conditions, they are not the primary site of inflammation in hypersensitivity vasculitis.

**Option B:** Arterioles are small blood vessels that regulate blood pressure and flow. While arterioles can be affected in conditions such as hypertension and atherosclerosis, they are not typically involved in hypersensitivity vasculitis.

**Option D:** Medium-sized arteries are not typically involved in hypersensitivity vasculitis. This condition primarily affects small blood vessels, such as postcapillary venules, rather than larger vessels like medium-sized arteries.

**Clinical Pearl / High-Yield Fact**
Hypersensitivity vasculitis can be triggered by various factors, including infections, medications, and autoimmune disorders. It is essential to consider these potential triggers when evaluating patients with symptoms of small vessel vasculitis.

**✓ Correct Answer:** C. Postcapillary venules