**Core Concept**
Hypersensitivity vasculitis, also known as leukocytoclastic vasculitis, is a type of small vessel vasculitis characterized by inflammation of blood vessels. It typically affects post-capillary venules, which are small veins that connect arterioles to capillaries.

**Why the Correct Answer is Right**
The correct answer is post-capillary venules because they are the primary site of inflammation in hypersensitivity vasculitis. This is due to the unique structure of post-capillary venules, which have a thin wall and a high concentration of immune complexes. The interaction of immune complexes with the endothelial cells of post-capillary venules triggers an inflammatory response, leading to the characteristic histopathological findings of leukocytoclastic vasculitis. The involvement of post-capillary venules is thought to be due to the presence of high levels of complement components, particularly C3 and C5, which are activated in this region.

**Why Each Wrong Option is Incorrect**
**Option A:** Aerioles are not typically involved in hypersensitivity vasculitis. While arterioles can be affected in other types of vasculitis, such as polyarteritis nodosa, they are not characteristically involved in hypersensitivity vasculitis.
**Option C:** Capillaries are not the primary site of inflammation in hypersensitivity vasculitis. While capillaries can be affected in some cases, the hallmark of hypersensitivity vasculitis is the involvement of post-capillary venules.
**Option D:** Medium-sized arteries are not typically involved in hypersensitivity vasculitis. This type of vasculitis typically affects small vessels, including arterioles, capillaries, and post-capillary venules.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hypersensitivity vasculitis typically presents with palpable purpura, which is a characteristic skin manifestation of this condition. The presence of palpable purpura should prompt a clinical suspicion of hypersensitivity vasculitis, particularly in patients with a history of recent infections or exposure to medications.

**✓ Correct Answer: B. Post capillary venules**