A 28-year-old male reports a 1-week history of fatigue, cough, and shortness of breath. Two days ago, he began coughing up blood-streaked sputum and noticed dark, tea-colored urine. He denies recent fever or upper respiratory symptoms but mentions mild weight loss over the last month. There is no history of recent travel or sick contacts. The patient is a smoker (1 pack/day for 5 years) and works in construction. He denies drug use or recent exposure to chemicals. Past Medical History: No known chronic conditions No previous episodes of hemoptysis or kidney problems Medications: None Family History: No significant history of autoimmune diseases or kidney problems Physical Examination: • Vital Signs: BP 160/95 mmHg, HR 105 bpm, RR 24/min, SpO₂ 91% on room air • General: Mildly pale, fatigued • Respiratory: Bilateral crackles at lung bases • Cardiovascular: Tachycardia without murmurs • Abdomen: No tenderness or organomegaly • Extremities: No edema • Skin: No rashes Initial Laboratory Results: • Hemoglobin: 9.0 g/dL (low) • Hematocrit: 28% • Platelet count: Normal • Serum creatinine: 3.0 mg/dL (elevated) • BUN: 45 mg/dL (elevated) • Urinalysis: Hematuria, proteinuria, red blood cell casts • Serum anti-GBM antibodies: Positive • ANA, ANCA: Negative Imaging: • Chest X-ray: Bilateral alveolar infiltrates consistent with pulmonary hemorrhage • Renal ultrasound: Normal-sized kidneys Renal Biopsy: • Light microscopy: Crescent formation in glomeruli • Immunofluorescence: Linear deposition of IgG along the glomerular basement membrane Which of the following best explains the mechanism of this disease?
