**Core Concept**
Wegener's granulomatosis, now known as **Granulomatosis with Polyangiitis (GPA)**, is a form of vasculitis that affects small- and medium-sized vessels. The underlying principle involves an autoimmune response leading to inflammation of blood vessels. This condition is characterized by the presence of **anti-neutrophil cytoplasmic antibodies (ANCA)**.

**Why the Correct Answer is Right**
The treatment of choice for GPA typically involves immunosuppressive therapy to reduce the autoimmune response and inflammation. This often includes a combination of **cyclophosphamide** and **corticosteroids** like **prednisone** for induction of remission, especially in severe cases. **Rituximab**, a **monoclonal antibody** targeting **CD20** on B cells, has also emerged as a effective treatment option, particularly for patients who are refractory to or cannot tolerate traditional immunosuppressants.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, while **corticosteroids** are used, they are typically part of a combination therapy rather than the sole treatment of choice.
**Option B:** Incorrect as it is not specified, but generally, **cyclophosphamide** alone without mentioning combination or context is not the complete answer.
**Option C:** Incorrect because, although **plasmapheresis** may be used in certain cases, it is not the primary treatment of choice for GPA.
**Option D:** Incorrect as it is not provided, but any option not including a combination of immunosuppressants or biologics like **rituximab** would be incomplete.

**Clinical Pearl / High-Yield Fact**
Remember, the treatment of GPA is tailored to the severity of the disease and the patient's response. **Rituximab** has become a crucial option, especially for relapsing or refractory disease, offering an alternative to traditional immunosuppressants.

**Correct Answer:** D. Cyclophosphamide and Corticosteroids.