A 25-year-old woman complains of low-grade fever, fatigue, and persistent rash over her nose and upper chest. She also notes pain in her knees and elbows. A skin biopsy shows dermal inflammation and granular deposits of IgG and C3 complement along the basement membrane at the epidermal/dermal junction. Urinalysis reveals microscopic hematuria and proteinuria. The ANA is positive. The development of thromboembolic complications (e.g., deep venous thrombosis) in this patient is commonly associated with elevated serum levels of antibodies to which of the following antigens?