You are answering a USMLE-style medical question. Question: A 75-year-old woman is brought to the emergency department by her son because of a 2-day history of fever, headache, malaise, and dry cough. The patient lives with her son and his family and her son reports that other members of the family have had similar symptoms during the past week but that he has not had any symptoms. The result of a polymerase chain reaction test confirms that the woman is infected with a virus that has a helical capsid and a segmented genome. Which of the following is the most likely explanation for the son being free of symptoms? Options: A) Serum antibodies against hemagglutinin B) Downregulation of ICAM-1 expression C) Protease-resistant antibodies in nasal secretions D) Homozygous mutation in the CCR5 gene Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.