## **Core Concept**
Good-Pasture Syndrome is a rare autoimmune disorder characterized by the presence of anti-glomerular basement membrane (anti-GBM) antibodies. It primarily affects the kidneys and lungs. The condition is named after the nephrologist William Goodpasture.

## **Why the Correct Answer is Right**
The correct answer, **C. Anti-GBM antibodies**, is right because Good-Pasture Syndrome is specifically defined by the presence of these antibodies. These antibodies target the glomerular and alveolar basement membranes, leading to rapidly progressive glomerulonephritis and pulmonary hemorrhage. The anti-GBM antibodies are pathogenic and directly cause the damage seen in Good-Pasture Syndrome.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Good-Pasture Syndrome does not primarily involve **Rheumatoid factor**, which is more commonly associated with rheumatoid arthritis and other autoimmune conditions. While rheumatoid factor can be present in various autoimmune diseases, it is not specific or diagnostic for Good-Pasture Syndrome.
- **Option B:** **ANCA (Anti-Neutrophil Cytoplasmic Antibodies)** are associated with conditions like Granulomatosis with Polyangiitis (formerly known as Wegener's granulomatosis) and Microscopic Polyangiitis, not Good-Pasture Syndrome. ANCA-associated vasculitides can present with similar symptoms but have a different pathophysiology.
- **Option D:** While **Lupus anticoagulant** is an antibody associated with Systemic Lupus Erythematosus (SLE) and antiphospholipid syndrome, it is not characteristic of Good-Pasture Syndrome.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Good-Pasture Syndrome often presents with a combination of renal and pulmonary symptoms, such as rapidly progressive glomerulonephritis and pulmonary hemorrhage. Early diagnosis and treatment, usually involving plasmapheresis and immunosuppression, are critical to improve outcomes.

## **Correct Answer: C. Anti-GBM antibodies**