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  3. Alzheimer’s disease is a problem of old age. In America, there is about a 50% chance that an individual will have developed it b

Alzheimer’s disease is a problem of old age. In America, there is about a 50% chance that an individual will have developed it b

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问题     Alzheimer’s disease is a problem of old age. In America, there is about a 50% chance that an individual will have developed it by the age of 85. As people live longer, its prevalence rises, and so does the incentive to find out how it works and treat it. But although the changes in the brain that accompany Alzheimer’s memory loss are being disentangled, the underlying cause remains mysterious.
    Most research has proceeded on the assumption that this cause is internal—either genetic or a side effect of the general wear and tear that accompanies ageing. But a few years ago, Brian Balin of the Philadelphia College of Osteopathic Medicine looked at the brains of 19 people who had died of Alzheimer’s and compared them with 19 people of the same age who had died of other causes. All but two of the Alzheimer’s brains had something in common; they harbored a bacterium known as Chlamydia pneumonia. Only one of the non-Alzheimer’s brain had this bug in it. That observation raised the possibility that sometimes, at least, Alzheimer’s disease is caused by an infection. This is a controversial, i-dea. But research that is about to be published in the journal Neurobiology of Aging, by Dr. Balin and his colleagues, suggests it might be right.
    The problem with the original observation was that Chlamydia is maddeningly common. Typically, the bug causes mild pneumonia, a persistent cough and a low-grade fever. Half of all people in their 20s have been exposed to it, and almost everyone has crossed the bacterium’s path by the time they reach old age. Finding evidence of Chlamydia in brain tissue from an autopsy therefore means little. Nor does the apparent correlation with Alzheimer’s disease prove anything. It might simply be that brains which are riddled with the "amyloid" protein plaques which characterize the disease are also more susceptible to infection.
    However, the idea that common pathogens are the culprits behind several diseases that have not previously been thought of as infections is gaining ground. A bacterium called Helicobacter pylori, for example, is now believed to be the main cause of stomach ulcers. And Chlamydia itself has possible (though disputed) links with heart disease.
    To gather evidence that Chlamydia might be causing Alzheimer’s disease, rather than merely profiting from it, Dr. Balin decided to see if the bug could induce Alzheimer’s-like symptoms in mice. He and his colleagues chose a kind of mouse that does not naturally develop amyloid brain-plaques. They then sprayed a strain of the bacterium that had been harvested from human Alzheimer’s patients up the mices noses. One to three months after doing so, Dr. Balin killed his mice and examined their brains. All the animals had developed plaques. Moreover, the longer an animal lived after the initial infection, the more plaques it had, and the larger they were.
What makes Balin’s hypothesis controversial is that______.
选项 A、the number of people they had examined is too limited to have general significance
B、the disease-causing bacterium is too common to be related to Alzheimer’s disease
C、the ages of the people examined, did not cover a range big enough for a conclusion
D、the original observations have already rooted out the possibility of infection
答案B
解析 细节题。第二段提到了巴林的研究结果，第三段指出，以前的观察认为衣原体是极常证明不了什么。这说明，人们之所以对巴林的假设提出疑问，是因为他们认为，可能引发老年痴呆症的衣原体病菌太常见，不可能与老年痴呆症有关系。B符合题意，正确选项。A和C是误解了文意；D与第三段第一句话的意思不符。
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