When fish or mammals ingest the chemical rotenone, enzymes in the

Question

When fish or mammals ingest the chemical rotenone, enzymes in the digestive tract metabolize most of it, rendering it harmless, but if enough rotenone enters the bloodstream, it can kill them. Wildlife managers who use rotenone in rivers or lakes to reduce fish populations claim that the practice is harmless to aquatic mammals, but clearly if enough rotenone is used to kill fish, mammals must be at risk too.

Which of the following, if true, most seriously weakens the argument?

A. Rotenone can enter a fish's bloodstream directly through the gills.
B. If populations of some fish species are allowed to grow unchecked, they can have disruptive ecological effects that ultimately harm wild mammal populations.
C. When rotenone is introduced into a fast-flowing river, it can travel a considerable distance downstream before breaking down.
D. For most aquatic mammals, fish constitute a substantial portion of their diet.
E. Chemicals that in the past have been used to reduce fish populations have considerably higher toxicity to aquatic mammals than rotenone does.

https://gmatclub.com/forum/download/file.php?id=76101

Contropositive · Accepted Answer

Hi  MartyMurray 
I'm not sure that I agree with above explanation. I think, (A) is not even remotely close. I would request you to review my below logical rejection for (A).

Premise:  When fish or mammals ingest the chemical rotenone, enzymes in the digestive tract metabolize most of it, rendering it harmless, but if enough rotenone enters the bloodstream, it CAN kill them

Conclusion:   if enough rotenone is used to kill fish, mammals MUST be at risk too.

Two things i noted:
1. Argument goes from CAN to MUST
2. Here, no comparison has been made between the ''risk'' if enough rotenone is used. 
 
This seems to be a ''so what?'' answer.  I need an option which says that:  Hey, mammals may not be at risk 
All this option says is ''Rotenone CAN enter fish's bloodstream directly. Does it say that, Rotenone enters directly more easily in Fish than in mammals? I don't think so.

Just because fish will be killed immediately, doesn't mean mammal may not be at risk.

MartyMurray · Answer

When fish or mammals ingest the chemical rotenone, enzymes in the digestive tract metabolize most of it, rendering it harmless, but if enough rotenone enters the bloodstream, it can kill them. Wildlife managers who use rotenone in rivers or lakes to reduce fish populations claim that the practice is harmless to aquatic mammals, but clearly if enough rotenone is used to kill fish, mammals must be at risk too.

The conclusion of the argument is the following:

if enough rotenone is used to kill fish, mammals must be at risk too

Not much support for the conclusion is provided, but the passage does say the following:

When fish or mammals ingest the chemical rotenone ... if enough rotenone enters the bloodstream, it can kill them.

We see that the reasoning of the argument is basically that, since rotenone can kill fish or mammals if enough enters their bloodstreams, if there's enough rodenone around to kill fish by entering their bloodstreams, there's also enough to do the same to mammals.

Which of the following, if true, most seriously weakens the argument?

This is a Weaken question, and the correct answer will cast doubt on the conclusion.

A. Rotenone can enter a fish's bloodstream directly through the gills.

This choice is interesting. After all, if rotenone can enter a fish's bloodstream directly through the gills, then maybe it's much easier for rotenone to kill fish than it is for it to harm mammals. After all mammals don't have gills. So, since rotenone can get into fish's bloodstreams in this way that's unique to fish, it's possible that rotenone would kill fish while being harmless to mammals.

So, while this choice doesn't prove that mammals won't be at risk if enough rotenone to kill fish is used, it does cast doubt on the conclusion "clearly if enough rotenone is used to kill fish, mammals must be at risk too." After all, if it's even possible that there's a key difference between fish and mammals that allows rotenone to kill fish without harming mammals, then its not clear tha mammals "must" be at risk.

So, this choice seems to be correct.

At the same time, it makes sense to be a little cautious with this choice since. After all, for this choice to be correct, we have to decide without being told that rotenone has no easy way to get into the bloodstreams of mammals.

Yes, we can use common knowledge in evaluating CR answer choices, but does common knowledge include knowing that mammals don't have gills and that mammals don't have anything like gills through which rotenone could enter their bloodstreams? Probably, but this choice is somewhat unique in how far it takes the common knowledge aspect of CR.

So, we can keep this choice while being prepared for the possibility of seeing a better one that even more clearly weakens the argument.

Keep.

B. If populations of some fish species are allowed to grow unchecked, they can have disruptive ecological effects that ultimately harm wild mammal populations.

This information indicates that using rotenone to control fish populations can benefit mammals.

At the same time, the fact that the practice of using rotenone can benefit mammals doesn't mean that rotenone doesn't also harm mammals. So, this choice doesn't cast doubt on the conclusion.

Eliminate.

C. When rotenone is introduced into a fast-flowing river, it can travel a considerable distance downstream before breaking down.

If anything, this choice strengthens, rather than weakens, the case for the conclusion. After all, if rotenone can travel a considerable distance before breaking down, then there may be many opportunities for it to harm aquatic mammals.

Eliminate.

D. For most aquatic mammals, fish constitute a substantial portion of their diet.

If anything, this choice strengthens, rather than weakens, the case for the conclusion.

After all, if fish constitute a substantial portion of the diet of most aquatic mammals, then if rotenone is getting into fish's bloodstreams, it may also get into the bloodstreams of aquatic mammals when they eat fish. So, by eating fish, aquatic mammals may consume enough rotenone to be harmed by it.

Eliminate.

E. Chemicals that in the past have been used to reduce fish populations have considerably higher toxicity to aquatic mammals than rotenone does.

This choice presents an irrelevant comparison. After all, regardless of whether other chemicals are more toxic to mammals than rotenone, it could still be the case that rotenone will harm aquatic mammals. In other words, the fact that something else is worse doesn't mean that rotenone is harmless.

Eliminate.

Since none of the other choices weaken the argument at all, the correct answer must be (A).

DmitryFarberMPrep · Answer

I'm inclined to agree with  Contropositive . This is not a great question. Knowing that rotenone CAN enter the bloodstreams of fish in a certain way doesn't tell us anything about whether it will enter the bloodstreams of mammals in some other way. I don't mind granting the common knowledge that fish have gills and mammals don't, but A doesn't really undermine the idea that mammals are not at risk. If it said that rotenone can ONLY be absorbed through the gills, that would be another story.

MartyMurray · Answer

Hi Contropositive.

We could argue that this question is not ideal, but it is gettable.

Since mammals don't have gills or anything similar to gills, the fact that rotenone can enter a fish's bloodstream directly through the gills by default means that rotenone can likely get into fish's bloodstreams more easily than it can into mammals' bloodstreams.

So, choice (A) casts doubt on the conclusion that "if enough rotenone is used to kill fish, mammals must be at risk too." After all, if rotenone can get into fish's bloodstreams in a way that's unique to fish, then we can say,  "Hey, mammals may not be at risk"  even if enough rotenone is used to kill fish.

We don't know for sure from (A) that mammals are not at risk of being harmed by enough rotenone to kill fish, but (A) does make it possible.

So, while (A) doesn't prove the conclusion wrong, it does cast doubt on it.

jdoe123 · Answer

MartyMurray  Just adding on here from your assessment of E below

Why would E be an irrelevant comparison? If other chemicals have a higher toxicity, that implies rotenone is less toxic, not harmless as you claim. 
By that yardstick, having rotenone be able to enter a fish via the gills doesnt mean it won't harm mammals as well...

MartyMurray · Answer

I didn't claim that it implies that rotenone is harmless.
 
Take a look at what you wrote and what I wrote.

You said basically the same thing I said.

Goldenfuture · Answer

I agree on this point. Also A leaves to many things to assumption. And what I have learned from so many CR questions is - pick a weakener which directly weakens conclusion. Does not leave it to assumption. E is better - something is in use in past and still something - river and aquatic life is alive - so it makes a better choice KarishmaB Bunuel - what's your logic?

egmat · Answer

The Argument: 
"If rotenone kills fish → it must kill mammals too"

PRETHINK (before looking at options):

What's the assumption? 
Fish and mammals are  equally vulnerable  to rotenone.

What would weaken? 
Something showing fish have a  UNIQUE weakness  mammals don't share.

Now I'm hunting for:  Different biology, different exposure, different entry points.

Scan Options With Prethink Filter:

(A) Rotenone enters fish bloodstream through gills →   ✓ MATCH   — Fish have gills, mammals don't
(B) Unchecked fish harm mammals → ✗ Ecology, not toxicity
(C) Rotenone travels downstream → ✗ Affects both equally
(D) Mammals eat fish → ✗ Would STRENGTHEN (more exposure!)
(E) Other chemicals more toxic → ✗ Compares to  other chemicals , not fish vs mammals

Only (A) matches our prethink.

Why (A) is a SLAM DUNK:

The passage says: "If rotenone  enters the bloodstream  → it can kill"

(A) says: Fish have  gills  → rotenone enters bloodstream  DIRECTLY

Connect the dots:

FISH: Gills → Bloodstream →   DEAD  
MAMMALS: Mouth → Stomach → Enzymes neutralize →   SAFE

Fish have a backdoor (gills) that mammals don't. 
Same poison. Different entry points. Different outcomes.

Why (E) is a TRAP:

What you THINK (E) says:  "Rotenone was used before, mammals survived"

What (E) ACTUALLY says:  "Other chemicals are MORE toxic than rotenone"

The fatal flaw:

Cyanide: Kills at  1 mg
Arsenic: Kills at  5 mg

"Arsenic is less toxic than cyanide!"
... Cool. Arsenic still kills you.

"Less toxic than X" ≠ "Safe"

And more importantly:  (E) compares rotenone to  other chemicals  — it doesn't address fish vs mammals AT ALL. Completely off-topic from what we need.

Adit_ · Answer

One can also make a point for mammals that way then right? What if mammals have something unique lets say the skin type of mammal that is diff from fishes and its unique to mammals and hence puts it at risk too. There is no mention of how the mammals possibly get affected in comparison to the fish in order to deem the conclusion to weaken which IMO says > "this then that" 
Answer strengthens "this" doesnt mean "that" does not happen because "this" is sttengthened IMO.
Please correct me if I am wrong.

MartyMurray · Answer

To disprove the conclusion, we'd need more information than is provided.

At the same time, a weakener has only to cast doubt on the conclusion. In other words, it doesn't have to cover all possibilities and prove the conclusion untrue.

So, choice (A) works because the fact that mammals don't have gills and fish do means that it's quite possible that the practice is indeed harmless to mammals and that the conclusion is not true.

Utsharma · Answer

MartyMurray

But we are assuming so many things for us to prove A correct. What if there are other ways in which that chemical enter a mammal's bloodstream. There have been so many CR questions in which this exact logic has been used to reject an option. Seems contradictory .

When fish or mammals ingest the chemical rotenone, enzymes in the

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