This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
[{'content': 'Which of the following being is the most frequent site of thrombus? \nSelect and explain. \nA. Veins of lower extremities, B. Portal vein, C. Pulmonary vein, D. Hepatic vein', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]