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
Dengue hemorrhagic fever - 
Select and explain. 
A. Most common in previously affected patients, B. DHF-2 most common cause, C. Immunosuppressed, D. Incubation period 7-14 days
A: Most common in previously affected patients
Answer: A: Most common in previously affected patients 
Explanation: None