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 Post subject: GMAT Verbal
PostPosted: Wed Aug 22, 2012 9:32 am 
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The view that the primary reason for the difficulty in isolating the causative organism of Legionnaire's disease was the organism's resistance to growth in culture on standard laboratory media had prevailed for many years. Scientists have been unable to prove the organism's infectivity because samples could not be isolated from diseased individuals, grown in the laboratory in pure culture, and then used to re-infect an experimental animal, thus establishing causation. An alternative view suggests that the primary obstacle to isolation of the Legionella bacterium is scientists' inability to view it microscopically. The conventional staining technique employed for creating bacterial samples was ineffective with the Legionella bacterium; although there was circumstantial evidence of an organism present on specimen slides, the lack of contrast between the unstained organism and the glass slide itself rendered the bacterium practically invisible.

Although no conclusive evidence exists to prove unequivocally that either theory is correct, many scholars believe that both are viable explanations. However, articles in public health literature have argued that the microscopic theory is correct exclusively. Unable to characterize a sample they could not view on a slide, epidemiologists investigated potentially non-biological causes of Legionnaire's disease, such as nickel poisoning. Finally, after ruling out other causes, they also devoted considerable time and resources to identifying unknown viruses that may have acted as causative agents.

However, pinning down the precise cause of a new disease is always full of pitfalls and wrong turns, and it has proved difficult to answer major questions concerning the exact importance of individual missteps. An additional difficulty is that experimental verification of either theory remains impossible. While the culture growth theory will always have its adherents, other theories abound.

The passage suggests which of the following about bacteria studied microscopically?
A. They are likely to be misidentified several times before scientists reach a consensus.
B. They cannot be developed in such a way to infect a laboratory animal.
C. They are difficult to identify unless highlighted by conventional staining techniques.
D. They are less likely to be the cause of a disease than bacteria grown on standard laboratory media.
E. They are more challenging to analyze when placed on glass slides than on slides made of other materials.


(C) The specific bacteria discussed in the passage that is studied microscopically is the Legionella bacterium. Scientists failed to identify it for some time because conventional staining techniques did not 'take up' the bacteria, making it difficult to identify against the backdrop of a glass slide. Choice (C) is correct, as suggested by the relevant sentence in the first paragraph.

While the causes of Legionnaire's were misidentified several times, we don't know whether that's generally true, eliminating (A).

(B) confuses the microscopic theory with the first theory presented at the beginning of the passage, which is the same as the problem with (D).

(E) focuses too much on the glass slides; the difficulty in identifying Legionella had more to do with the staining process than any details in the makeup of the slides.
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Regarding answer C :
If "scientists failed to identify it for some time because conventional staining techniques did not 'take up' the bacteria", why would the answer C, "they are difficult to identify UNLESS highlighted by conventional staining techniques" be the proper answer ?

In other words, if the conventional staining techniques did not prove to be efficient, why would we consider that these techniques would make it easier to identify the bacteria, as suggested by answer C ? ("unless highlighted by conventional staining techniques")

Thanks !


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 Post subject: Re: GMAT Verbal
PostPosted: Wed Aug 22, 2012 9:33 am 
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Joined: Thu Jul 05, 2012 11:55 am
Posts: 64
Quote:
Regarding answer C :
If "scientists failed to identify it for some time because conventional staining techniques did not 'take up' the bacteria", why would the answer C, "they are difficult to identify UNLESS highlighted by conventional staining techniques" be the proper answer ?

In other words, if the conventional staining techniques did not prove to be efficient, why would we consider that these techniques would make it easier to identify the bacteria, as suggested by answer C ? ("unless highlighted by conventional staining techniques")

Thanks !
The key here is that the question asks about "bacteria studied microscopically," not about Legionella specifically. The passage tells us that bacteria are difficult to identify on slides unless the conventional staining process results in a contrast between the bacterium and the slide, so Legionella is difficult to identify because staining it does not create this contrast. Choice (C) correctly describes "bacteria studied microscopically."


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