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: Medicine
[{'content': 'Dengue hemorrhagic fever - \nSelect and explain. \nA. Most common in previously affected patients, B. DHF-2 most common cause, C. Immunosuppressed, D. Incubation period 7-14 days', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]