Review Question - QID 108695

A 61-year-old man presents to the emergency room with a painful, swollen left leg. He states that his symptoms began that morning after a long flight from Australia. He denies shortness of breath, chest pain, or cough. On review of systems, he notes that he has been constipated recently and had several episodes of bright red blood per rectum. He has not noticed any weight loss, fevers, or night sweats. He has a past medical history significant for a deep venous thrombosis 4 years ago when he was hospitalized for community-acquired pneumonia and was treated with warfarin for 3 months afterward. He also has chronic hepatitis C from previous intravenous drug use. The patient has a 30-pack-year smoking history and has never had a colonoscopy. His father has chronic kidney disease from diabetes and his mother passed away from a massive pulmonary embolus when pregnant with his younger sister. In the emergency room, his temperature is 98.7°F (37.1°C), blood pressure is 142/85 mm/Hg, pulse is 79/min, and respirations are 14/min. On exam, he is in no acute distress. His left calf is red, tender to palpation, and larger in caliber than the right calf. Dorsiflexion of the left foot worsens the pain. His abdomen is soft, non-tender, and non-distended without hepatomegaly. The remainder of the physical exam is unremarkable. Labs are shown below:

Hemoglobin: 13.0 g/dL
Leukocyte count: 6,000/mm^3
Platelets: 160,000/mm^3

Aspartate aminotransferase: 15 U/L
Alanine aminotransferase: 19 U/L
Alkaline phosphatase: 81 IU/L
Hepatitis C antibody: reactive
Hepatitis C titer: 0 copies/mL

Which of the following is the most likely cause of this patient’s symptoms?