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Complete Question Explanation

(See the complete passage discussion here: lsat/viewtopic.php?t=14646)

Must Be True. The correct answer choice is (E)

In approaching this Must Be True question, we need to eliminate responses that are contrary to the facts
and inferences within the passage.

Answer choice (A): This answer choice states that nerve cells produce anti-NGF, and is therefore
unsupported by the passage, which never indicates precisely where anti-NGF is produced.

Answer choice (B): This choice asserts that cells not affected by NGF are less numerous than those
affected by NGF, and that the different cells have the same qualities. While the passage does support the
idea that there are different types of cells, not all of which are affected by NGF, but no further detail is
offered as to similarities or quantitative comparisons.

Answer choice (C): The passage does suggest that a significant number of nerve cells probably die off
in the process of an organism’s development. However, since the passage never offers any information
about the relative number of surviving cells, the conclusion that “few” cells are needed is unsupported.

Answer choice (D): Nothing in the passage indicates that some nerve cells have the capacity to change
into other types of living tissue, so this answer choice is incorrect.

Answer choice (E): This is the correct answer choice, as it is supported by the discussion in the
second paragraph. The passage states that an embryo initially produces more nerve cells than needed, and that the extra cells die off, which already supports the idea that the embryo produces nerve cells that
do not end up connecting to anything. Furthermore, the second paragraph indicates that NGF governs
the process by which some nerve cells develop and others die off (lines 14-26), and the third paragraph
explains, in detail, that NGF is the causal factor that helps direct nerve cells toward target cells. It is
reasonable to conclude that since some nerve cells will die off, they do not receive NGF, and do not
connect with target cells.
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2019 Bible Pg. 402 (Levi-Montalcini) - Question 18


I am not sure that I agree with your explanation of answer choice (C) of Question 18 in the 2019 LGB.
I understand the situation as: If there are 100 nerve cells, only, say, 50 of them are necessary to target cells, so only these are selected by NGF and are directed towards the target cells, while the other half die.
In your explanation of (C), you say: "since the passage never offers any information about the relative number of surviving cells," we cannot say that only "few" cells survive.
I think I see what you are saying: there can be 50, 60, or 70 surviving cells, so they are not necessarily 'few' in number.
But I think the 'few nerve cells' in answer choice (C) refers to a different group of nerve cells.
(C) says: "few of the nerve cells that connect with target cells..." (that is, few of the 50 cells that do connect to (that are necessary to) the target cells) "are needed by the organism..."
In other words, I think (C) can mean something like: only 3 of the 50 surviving nerve cells that are necessary to target cells are necessary to the organism itself.
Such a piece of information has not been given in the passage, so I think (C) is incorrect.
Please let me know whether my line of thought is correct.

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Hi Harry!

You are correct that any statement that is not supported by the passage would be an incorrect answer for this question. So it sounds like however you interpret answer choice (C), it is unsupported by the passage and therefore incorrect. I'm not sure the distinction you are trying to make is a necessary one--though I appreciate your close reading! Ultimately, the passage gives us no information about the relative number of cells that survive or the relative number that connect to their target cells. For all we know, all of the surviving cells connect to their target cells. But either way you look at it, the passage doesn't give you enough information to support answer choice (C) so it is definitely incorrect!

Hope this helps!

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Would C be valid if it said "Few of the nerve cells are needed by the organism in which they develop."?

When the passage says "many of the immature nerve cells produced in the development of an organism are normally programmed to die", does that means there are FEW immature nerve cells that are NOT normally programmed to die?
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I'm not sure if this is an adequate explanation for this question. On your explanation of C, the passage does provide information about the relative number of surviving cells. In lines 17-18, Levi-Montalcini's experiment finds that embryos produce "many more cells than are finally required," showing that few cells survive relative to the overall number that are produced.

On answer choice E, I'm not sure if the passage really does support this. Just because cells are produced which are destined to die does not mean those cells never connect to anything. In lines 51-55, the passage states that the two functions of NGF are to direct nerve cells toward target cells and "later being necessary for the continued survival of those nerve cells." This might mean that nerve cells connect with targets and then die off afterward if NGF disappears, so the mere fact that nerve cells die doesn't seem to entail those nerve cells never connected with a target cell. The passage implies the possibility of a connected cell later dying off in line 58 when the passage says the types of cells affected by NGF "die if the factor is not present."

The phrase "the types of nerve cells that are affected by NGF" I think is the best support for E as the correct answer, as this seems to imply there is a type of nerve cell not affected by NGF, and since NGF is necessary for nerve cells to reach their target cells, this other type of nerve cell does not connect with target cells. However, I'm still confused as to how C is not also supported. Maybe Harry's explanation is the only way to prove C wrong.
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As I see it, Luke, the biggest problem with answer C lies in its focus on those cells that actually connect with target cells. We know that in the embryo, many more cells are created than are required, but do we know whether any of the ones that eventually successfully connect with their targets are not needed? That is, we start off with an overabundance of nerve cells; some of them then get directed by NGF on where to go (NGF acting like a sort of traffic cop); and then...what? Could it be that 100% of the ones that connect are absolutely necessary for the organism? Could it be that the only unnecessary cells are the ones that never make those connections and end up dying off, leaving only the required number?

We have no information in the passage to support answer C. It's entirely speculative. Kill it off like a useless nerve cell!

Support in favor of answer E is, in my view, a little harder to pin down, and I would settle for it based on process of elimination alone, but there IS at least some support for it. It's in the idea that we start off with way more than needed, and the number eventually adjusts lower to match up with the tissues that need them. The excess MIGHT connect to some target cells, but there is at least an implication that they die off without ever having been used. They are unnecessary surplus, and once all the tissues have the connections they need, the rest of those nerve cells just die off without ever having done anything.

And one other thing: in the last paragraph, we see this:
NGF seems to play two roles, serving initially to direct the developing nerve processes toward the correct, specific "target" cells with which they must connect
This suggests that the extra cells are ones that DON'T have to connect to anything, and if they don't have to connect, they won't get sent by NGF to make those connections.

Is it an absolute guarantee? Nope, but that's not the standard to apply to a question like this, which is a Most Strongly Supported question, the "softer" variety of Must Be True that requires some support but not complete proof. The other answers have zero support while E has at least a little, making it the one choice that is "most" supported here.

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