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# The optic nerves of identical twins are genetically identical.

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Manager
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The optic nerves of identical twins are genetically identical.  [#permalink]

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18 Aug 2017, 23:09
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95% (hard)

Question Stats:

49% (02:36) correct 51% (02:23) wrong based on 215 sessions

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The optic nerves of identical twins are genetically identical. When only one of a set of identical twins is visually impaired, certain areas of the affected twin’s optic nerve are atrophied whereas the corresponding areas of the other twin’s optic nerve is unaffected. No such differences are found when neither twin is visually impaired. Therefore, this discovery proves definitively that blindness is caused by damage to the structure of the optic nerve.

The argument above depends upon which one of the following?

A.The optic nerve of a blind person is smaller than that of anyone not suffering from visual impairment.

B.The relative size of the optic nerve of visually impaired individuals is not the result of corrective treatments or medications used to treat visual impairment.

C.The optic nerve of a person with an identical twin is no larger, on average, than the optic nerve of a person who is not an identical twin.

D.When a set of identical twins are both visually impaired, their optic nerves are in the same relative condition.

E.Individuals who have an identical twin are no more likely to suffer from visual impairment than are those who do not.
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The optic nerves of identical twins are genetically identical.  [#permalink]

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18 Aug 2017, 23:40
Conclusion :- Therefore, this discovery proves definitively that blindness is caused by damage to the structure of the optic nerve.
If there is any alternative cause,this would not hold.

Check option B
The relative size of the optic nerve of visually impaired individuals is not the result of corrective treatments or medications used to treat visual impairment.
Negate this and you will get an alternate cause which destroys the argument.
Therefore B is correct.
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Re: The optic nerves of identical twins are genetically identical.  [#permalink]

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19 Aug 2017, 00:03
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Conclusion :- Therefore, this discovery proves definitively that blindness is caused by damage to the structure of the optic nerve.
Option B :-The relative size of the optic nerve of visually impaired individuals is not the result of corrective treatments or medications used to treat visual impairment.
This option shows that the shrinkage of the optic nerve of impaired persons is not due to an alternate cause i.e Corrective treatments or medications used to treat visual Treatment. The alternate cause is ruled out. This premise strengthens the conclusion that blindness is caused by damage to the optic nerve,not any other reason. Please give me kudos . I need them badly
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Re: The optic nerves of identical twins are genetically identical.  [#permalink]

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19 Aug 2017, 03:44
A. Irrelevant
B. Correctly assumes that gap between premises and the conclusion.
C. Irrelevant
D. This is not assumed but implied from the argument given.
E. Irrelevant

By assuming that the treatment of the visual impairment did not affect the optic nerve, it is clear that optic nerve plays a part in the visual impairment.
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Re: The optic nerves of identical twins are genetically identical.  [#permalink]

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19 Aug 2017, 13:53
The optic nerves of identical twins are genetically identical. When only one of a set of identical twins is visually impaired, certain areas of the affected twin’s optic nerve are atrophied whereas the corresponding areas of the other twin’s optic nerve is unaffected. No such differences are found when neither twin is visually impaired. Therefore, this discovery proves definitively that blindness is caused by damage to the structure of the optic nerve.

The argument above depends upon which one of the following?

A.The optic nerve of a blind person is smaller than that of anyone not suffering from visual impairment. -This is a fact set. Irrelevant

B.The relative size of the optic nerve of visually impaired individuals is not the result of corrective treatments or medications used to treat visual impairment. -Correct. This option removes the possibility of any alternate cause.

C.The optic nerve of a person with an identical twin is no larger, on average, than the optic nerve of a person who is not an identical twin. -This is a fact set. Irrelevant

D.When a set of identical twins are both visually impaired, their optic nerves are in the same relative condition. -This information is given in the premise. Even if this were to be correct, it doesn't give any information about the cause of blindness.

E.Individuals who have an identical twin are no more likely to suffer from visual impairment than are those who do not. -This is a fact set. Irrelevant
_________________
Manager
Status: Searching for something I've been searching..LOL
Joined: 14 Dec 2016
Posts: 58
Location: India
Concentration: Healthcare, Operations
Schools: Ross '20
GMAT 1: 590 Q35 V42
GPA: 3.5
WE: Medicine and Health (Health Care)
Re: The optic nerves of identical twins are genetically identical.  [#permalink]

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19 Aug 2017, 23:23
This is a causal argument. Since the optic nerve in the blind twin is smaller than that in the twin with normal vision, the argument concludes that damage to the optic nerve is the cause of blindness. To weaken this argument, we need to assert either that there is some other cause, or that the causality is reversed. Answer B asserts that the causality could be reversed: the small size of the optic nerve might be the result of the treatments given to the blind child. Therefore, the correct answer is B.

Subject Area: Critical Reasoning

Categories: Weaken
Re: The optic nerves of identical twins are genetically identical.   [#permalink] 19 Aug 2017, 23:23
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