**Core Concept**
Necrotizing arteriolitis with fibrinoid necrosis is a histopathological feature that occurs due to the deposition of antigen-antibody complexes in the small arteries and arterioles. This process leads to inflammation and damage to the vessel walls, resulting in necrosis.

**Why the Correct Answer is Right**
The correct answer is Ag-Ab complex mediated because the formation of antigen-antibody complexes is a key event in the pathogenesis of this condition. When antibodies bind to antigens, they can form immune complexes that deposit in the small arteries and arterioles. These immune complexes activate the complement system, leading to inflammation and damage to the vessel walls. The deposition of fibrinoid material in the vessel walls is a characteristic feature of this process. The involvement of immune complexes is a hallmark of type III hypersensitivity reactions, which include conditions such as systemic lupus erythematosus and rheumatoid arthritis.

**Why Each Wrong Option is Incorrect**
**Option A:** Immediate hypersensitivity reactions, also known as type I hypersensitivity reactions, are mediated by IgE antibodies and involve the release of histamine and other mediators from mast cells. This type of reaction is not associated with fibrinoid necrosis.

**Option B:** Cell-mediated immunity, also known as type IV hypersensitivity, involves the activation of T lymphocytes and macrophages, but it does not typically result in fibrinoid necrosis.

**Option D:** Cytotoxic mediated reactions involve the direct damage to cells by cytotoxic T cells or antibodies, but they do not typically result in fibrinoid necrosis.

**Clinical Pearl / High-Yield Fact**
Fibrinoid necrosis is a characteristic feature of vasculitis, which involves inflammation of the blood vessels. It is often seen in conditions such as systemic lupus erythematosus, rheumatoid arthritis, and polyarteritis nodosa.

**✓ Correct Answer: C. Ag-Ab complex mediated**