This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Virology, Subject name: Microbiology
On November 6, a patient had the onset of an illness characterized by fever, chills, headache, cough, and chest pain. The illness lasted 1 week. On December 5, she had another illness very similar to the first, which lasted 6 days. She had no influenza immunization during this period. Her hemagglutination inhibition antibody titers to nH1N1 influenza virus were as follows:November 6: 10 November 30: 10 December 20: 160There was no laboratory error. Which of the following is the best conclusion from these data? 
Select and explain. 
A. The patient was ill with influenza on November 6, B. The patient was ill with influenza on December 5, C. The patient was ill with influenza on December 20, D. It is impossible to relate either illness with the nH1N1 influenza virus
B: The patient was ill with influenza on December 5
Answer: B: The patient was ill with influenza on December 5 
Explanation: The symptoms described for her illnesses on both November 6 and December 5 are consistent with influenza. However, the November 6 illness (a) was not caused by nH1N1 influenza virus; the low titers (10 on November 6 and 10 on November 30) most likely represent cross-reacting antibodies from the agent that caused her first illness. The greater than fourfold rise in titer from 10 on November 30 (baseline) to 160 on December 20 reflects a definitive diagnostic rise in antibody against nH1N1 influenza virus. Thus, her December 5 illness (b) was influenza. There is enough serologic evidence to make the diagnosis (choice e is incorrect).