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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.
The example of Helicobacter pylori is mentioned in the text to______
选项 A、illustrate that Chlamydia may be the cause of heart disease
B、show that people used to misunderstand the cause of stomach ulcers
C、refer to the possible relationship between Chlamydia and Alzheimer
D、tell the readers that people are making progress in fighting infection
答案C
解析 局部主题题(例证题)。题干中的“Helicobacler pylori”出自文章第四段。第三段介绍了对巴林研究的怀疑，第四段转折指出，但常见的病原体是造成几种疾病的罪魁祸首，现在这种观点开始盛行，接着提到了幽门螺旋杆菌。这说明，提到该细菌是为了表明，衣原体可能与老年痴呆症有关系。C符合题意，为正确选项。A、B是作者所举的例子，不是例子要说明的问题：D是误解了作者的意图。
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