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  3. For some time scientists have believed that cholesterol plays a major role in heart disease because people with familial hyperch

For some time scientists have believed that cholesterol plays a major role in heart disease because people with familial hyperch

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问题     For some time scientists have believed that cholesterol plays a major role in heart disease because people with familial hypercholesterolemia, a genetic defect, have six to eight times the normal level of cholesterol in their blood and they invariably develop heart disease. These people lack cell-surface receptors for low-density lipoproteins (LDL’s), which are the fundamental carriers of blood cholesterol to the body cells that use cholesterol. Without an adequate number of cell-surface receptors to remove LDL’s from the blood, the cholesterol-carrying LDL’s remain in the blood, increasing blood cholesterol levels. Scientists also noticed that people with familial hypercholesterolemia appear to produce more LDL’s than normal individuals. How, scientists wondered, could a genetic mutation that causes a slowdown in the removal of LDL’s from the blood also result in an increase in the synthesis of this cholesterol-carrying protein?
    Since scientists could not experiment on human body tissue, their knowledge of familial hypercholesterolemia was severely limited. However, a breakthrough came in the laboratories of Yoshio Watanabe of Kobe University in Japan in 1980. Watanabe noticed that a male rabbit in his colony had ten times the normal concentration of cholesterol in its blood. By appropriate breeding, Watanabe obtained a strain of rabbits that had very high cholesterol levels. These rabbits spontaneously developed heart disease. To his surprise, Watanabe further found that the rabbits, like humans with familial hypercholesterolemia, lacked LDL receptors. Thus, scientists could study these Watanabe rabbits to gain a better understanding of familial hypercholesterolemia in humans.
    Prior to the breakthrough at Kobe University, it was known that LDL’s are secreted from the liver in the form of a precursor, called very low-density lipoproteins (VLDL’s) , which carry triglycerides as well as relatively small amounts of cholesterol. The triglycerides are removed from the VLDL’s by fatty and other tissues. What remains is a remnant particle that must be removed from the blood. What scientists learned by studying the Watanabe rabbits is that the removal of the VLDL remnant requires the LDL receptor. Normally, the majority of the VLDL remnants go to the liver where they bind to LDL receptors and are degraded. In the Watanabe rabbit, due to a lack of LDL receptors on liver cells, the VLDL remnants remain in the blood and are eventually converted to LDL’s. The LDL receptors thus have a dual effect in controlling LDL levels. They are necessary to prevent oversynthesis of LDL’s from VLDL remnants and they are necessary for the normal removal of LDL’s from the blood. With this knowledge, scientists are now well on the way toward developing drugs that dramatically lower cholesterol levels in people afflicted with certain forms of familial hypercholesterolemia.
In which of the following ways does the passage imply that Watanabe rabbits differ from normal rabbits?
选项 A、Watanabe rabbits have more LDL receptors than do normal rabbits.
B、The blood of Watanabe rabbits contains more VLDL remnants than does the blood of normal rabbits.
C、Watanabe rabbits have fewer fatty tissues than do normal rabbits.
D、Watanabe rabbits secrete lower levels of VLDL’s than do normal rabbits.
答案B
解析 事实细节题。第二段第三句指出，Watanabe发现一只公兔子血液中的胆固醇浓度是标准浓度的十倍。通过第六句可知，Watanabe所饲养的这些兔子缺少低密度脂蛋白受体。而通过第三段第四句可知，低密度脂蛋白受体又是去除超低密度脂蛋白残留物所必需的。结合这三句内容可知，Watanabe饲养的兔子比普通兔子缺少低密度脂蛋白受体，从而其血液中的超低密度脂蛋白残留物要多于其他兔子，故答案为[B]项。
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