**Core Concept**
Wegener's granulomatosis, now referred to as Granulomatosis with Polyangiitis (GPA), is a form of vasculitis that affects small- and medium-sized vessels. This autoimmune disease is characterized by the presence of granulomatous inflammation and necrotizing vasculitis, primarily involving the respiratory tract and kidneys.

**Why the Correct Answer is Right**
The respiratory tract, particularly the lungs and trachea, are often the initial sites of involvement in GPA. The granulomatous inflammation in these areas can lead to symptoms such as cough, hemoptysis, and dyspnea. The kidneys are also frequently affected, resulting in glomerulonephritis and potentially progressing to renal failure. The combination of respiratory and renal involvement is a hallmark of GPA.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because, while GPA can involve other organs such as the skin, joints, and eyes, they are not the most common sites of involvement.
* **Option B:** This option is incorrect because, although GPA can involve the gastrointestinal tract, it is not a common site of involvement compared to the respiratory tract and kidneys.
* **Option C:** This option is incorrect because, while GPA can involve the central nervous system, it is not a common site of involvement.

**Clinical Pearl / High-Yield Fact**
GPA often presents with a triad of symptoms: respiratory symptoms (e.g., cough, hemoptysis), renal symptoms (e.g., hematuria, proteinuria), and systemic symptoms (e.g., fever, weight loss). Recognizing this triad is crucial for early diagnosis and treatment.

**Correct Answer:** D.