**Core Concept**
The patient's symptoms and lab findings suggest a diagnosis of **Wegener's granulomatosis**, now known as **Granulomatosis with Polyangiitis (GPA)**, a type of **anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis**. This condition is characterized by **necrotizing vasculitis** and **granuloma formation**.

**Why the Correct Answer is Right**
The first-line treatment for GPA involves **immunosuppressive therapy** to reduce inflammation and prevent organ damage. **Cyclophosphamide** and **Rituximab** are commonly used, but the initial treatment often starts with **Corticosteroids** and an **immunosuppressant** like **Cyclophosphamide** or **Methotrexate** for remission induction. However, given the severity and the need for rapid control, **Cyclophosphamide** is frequently chosen for its potent immunosuppressive effects.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, while **Corticosteroids** are used, they are often combined with another immunosuppressant for GPA treatment.
**Option B:** Incorrect as it is not specified, but typically, **Cyclophosphamide** or **Rituximab** would be considered over other options for GPA.
**Option D:** Incorrect because, although **Methotrexate** can be used in GPA treatment, it's not typically the first choice for severe, active disease.

**Clinical Pearl / High-Yield Fact**
Remember, GPA is characterized by the presence of **C-ANCA** (cytoplasmic anti-neutrophil cytoplasmic antibodies) and **necrotizing granulomas**, particularly in the respiratory tract, and **vasculitis** affecting small to medium-sized vessels. Treatment should be initiated promptly to prevent organ damage.

**Correct Answer:** D. Cyclophosphamide