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: Surgery
[{'content': 'Gold standard diagnostic test in varicose veins is \nSelect and explain. \nA. Photoplethysmography, B. Duplex imaging, C. Ultrasonography, D. Radio - labeled fibrinogen study', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]