**Core Concept**
Wagner's granulomatosis, also known as granulomatosis with polyangiitis (GPA), is a form of vasculitis that affects small- and medium-sized vessels. It is characterized by the presence of granulomas and necrotizing vasculitis, primarily affecting the respiratory tract and kidneys.

**Why the Correct Answer is Right**
The correct answer is related to the clinical manifestations of Wagner's granulomatosis. GPA typically affects the respiratory tract, including the sinuses, trachea, and bronchi, as well as the kidneys, leading to symptoms such as cough, hemoptysis, and renal failure. The disease process involves the deposition of autoantibodies against neutrophil cytoplasmic antigens (ANCA), leading to the activation of neutrophils and the subsequent formation of granulomas and necrotizing vasculitis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the respiratory tract is a common site of involvement in GPA.
**Option B:** This option is incorrect because the kidneys are frequently affected in GPA, leading to renal failure.
**Option C:** This option is incorrect because GPA can also affect the skin, causing symptoms such as purpura and ulcers.

**Clinical Pearl / High-Yield Fact**
GPA is a classic example of an ANCA-associated vasculitis, and the presence of ANCA is a key diagnostic criterion. The disease is typically treated with a combination of immunosuppressive medications, including glucocorticoids and cyclophosphamide.

**Correct Answer: C. Skin**