The following are multiple choice questions (with answers) about medical knowledge.   **Question:** A 21-year-old man comes to the emergency room with swelling and severe pain in his left lower leg that started 2 hours ago. He has no history of serious illness or trauma. His father has a history of pulmonary embolism. He has smoked one pack of cigarettes daily since he started college 3 years ago. He appears very distressed. He is 173 cm (5 ft 8 in) tall and weighs 92 kg (203 lb); BMI is 30.7 kg/m2. His temperature is 37°C (98.6°F), pulse is 94/min, respirations are 17/min, and blood pressure is 130/78 mm Hg. Physical examination shows a tender and mildly swollen left lower leg; dorsiflexion of the left foot causes severe pain in the calf. Laboratory studies show a platelet count of 184,000/mm3, a prothrombin time of 11 seconds, an activated partial thromboplastin time of 26 seconds, and positive fibrin split products. Ultrasonography of the left leg shows incompressibility of the popliteal vein with a hyperechoic mass and absent blood flow. The patient is administered a 5000 IU intravenous bolus of unfractionated heparin followed by a constant infusion. Six hours later, the activated partial thromboplastin time is 30 seconds. Which of the following is the most likely cause of this patient's symptoms?   (A) Antithrombin III deficiency   (B) Factor V Leiden   (C) Protein C deficiency   (D) Antiphospholipid syndrome
['(A) Antithrombin III deficiency <> answer', 'The following are multiple choice questions (with answers) about medical knowledge. <> instruction', '(A) Antithrombin III deficiency \n (B) Factor V Leiden \n (C) Protein C deficiency \n (D) Antiphospholipid syndrome <> possible answers', "  A 21-year-old man comes to the emergency room with swelling and severe pain in his left lower leg that started 2 hours ago. He has no history of serious illness or trauma. His father has a history of pulmonary embolism. He has smoked one pack of cigarettes daily since he started college 3 years ago. He appears very distressed. He is 173 cm (5 ft 8 in) tall and weighs 92 kg (203 lb); BMI is 30.7 kg/m2. His temperature is 37°C (98.6°F), pulse is 94/min, respirations are 17/min, and blood pressure is 130/78 mm Hg. Physical examination shows a tender and mildly swollen left lower leg; dorsiflexion of the left foot causes severe pain in the calf. Laboratory studies show a platelet count of 184,000/mm3, a prothrombin time of 11 seconds, an activated partial thromboplastin time of 26 seconds, and positive fibrin split products. Ultrasonography of the left leg shows incompressibility of the popliteal vein with a hyperechoic mass and absent blood flow. The patient is administered a 5000 IU intravenous bolus of unfractionated heparin followed by a constant infusion. Six hours later, the activated partial thromboplastin time is 30 seconds. Which of the following is the most likely cause of this patient's symptoms? <> context@"]