A 62-year-old man with chronic obstructive pulmonary disease presents with acute exacerbation characterized by increased sputum purulence and dyspnea. Sputum culture is pending. He is started on levofloxacin 750 mg once daily. On day 3 of therapy, he develops acute onset severe left calf pain and swelling with palpable cord. Duplex ultrasound confirms deep vein thrombosis. What is the most appropriate next step in management?