A 28-year-old man has had hemoptysis and hematuria for the past 2 days. On physical examination, his temperature is 36.8deg C, pulse is 87/min, respirations are 19/min, and blood pressure is 150/90 mm Hg. Laboratory studies show creatinine of 3.8 mg/dL and urea nitrogen of 35 mg/dL. Urinalysis shows 4+ hematuria, 2 + proteinuria, and no glucose. A renal biopsy specimen examined microscopically shows glomerular damage and linear immunofluorescence with labeled complement C3 and anti-IgG antibody. Which of the following autoantibodies has the greatest specificity for this patient's condition?