A 26-year-old woman comes to the emergency complaining of 2 day of worsening right leg pain and swelling. She drove in a car 8 hours back from a hiking trip 2 days ago and then noticed some pain in the leg. At first she thought it was due to exeion, but it has worsened over the day. Her past medical history is related to difficulty getting pregnant, with two prior spontaneous aboions. Her physical examination is notable for normal vital signs, hea, and lung examination. Her right leg is swollen from the mid-thigh down and is tender. Doppler studies demonstrate a large deep venous thrombosis in the femoral and ileac veins extending into the pelvis. Laboratory studies show normal electrolytes, normal WBC and platelet counts, normal PT, and an aPTT 3x normal. Her pregnancy test is negative. Low-molecular-weight heparin therapy is initiated in the emergency depament. Subsequent therapy should include which of the following?