A 38-year-old woman with no significant past medical history presents to the emergency department with acute-onset left leg swelling, warmth, and erythema that began 6 hours ago. She denies recent travel, immobilization, or trauma. Vital signs are: BP 128/82 mmHg, HR 92 bpm, RR 16/min, temperature 37.2°C, and SpO2 98% on room air. Physical examination confirms unilateral lower extremity edema and pitting erythema over the medial thigh. Laboratory studies show D-dimer 1.8 mcg/mL (elevated). Compression ultrasound of the left lower extremity reveals echogenic material within the femoral vein with loss of compressibility and absent flow on Doppler assessment. The patient denies dyspnea, pleuritic chest pain, or hemoptysis. Which of the following is the most appropriate initial anticoagulation strategy?

1. A)Aspirin 325 mg daily for antiplatelet effect and close clinical observation
2. B)Warfarin monotherapy initiated immediately with target INR 2-3
3. C)Unfractionated heparin bolus followed by continuous infusion or weight-based low-molecular-weight heparinGABARITO
4. D)Apixaban 5 mg twice daily immediately without bridging anticoagulation
5. E)Fondaparinux monotherapy initiated without bridging to warfarin