【单选题】

The passage implies that research into the cause of human narcolepsy has been complicated by what aspect of the human brain
A．(A) The tendency of certain neurons to resemble foreign matter that HLAs reject

B．(B) A lack of apparent neurological damage caused by a tendency to remove debris through support cells
C．(C) Its susceptibility to diseases like Parkinson's and Alzheimer's
D．(D) Its tendency to reject the drugs which scientists have developed to treat narcolepsy's symptoms
E．(E) The effects of aging on the symptoms of the disease