#6 - One test to determine whether a person has been

[moshei24]

Is the reason the answer is (C) because if it wasn't exclusive to that bacterium, it wouldn't necessarily be that anyone who reacts to the injection is reacting from the injection? Rather, it could be something else that's causing the reaction and it just happened to occur at the same time as the injection?

I think I'm a little bit confused by the wording of (C). What does it mean when it says "it's exclusive to that bacterium"? That is what's really confusing me.

The argument seems to be saying that once someone is infected with TB, only then will they have the reaction from being injected with some proteins.

Can someone please clear this up for me? Thanks!

[Steve Stein]

In that one, the author points out that once a person has been infected by a bacterium, the immune system will react to certain proteins in that bacterium, causing irritation.

The doctors concluded that irritation meant a subject had been infected.

So, basically, the doctors are concluding that the proteins recognized are from TB--presuming, as the correct answer choice provides, that "the ability...to trigger the skin irritation is exclusive to that bacterium." That is, that no other bacteria share the same proteins.

I hope that clears that one up--let me know--thanks!

~Steve

[moshei24]

I'm confused. Aren't they injecting him with TB proteins? So why would it matter if TB has the same proteins as other bacteria?

[gen2871]

Hi Dear LSAT Masters:

I got C is the correct answer based on my diagram:

Inject bacterium immune system recognition certain protein and attack skin irritation

Skin irritation TB infection

But when I tried to use the assumption negation technique on E, I didnt know where to place my negation. The diagram yield from the answer choice ought to be

recognize infectious bacteria sufficient quantities of bacteria to cause overt symptoms of disease.

I understand this cannot be the correct answer from reading this answer choice, but I wonder how should I do if I were to negate a conditional like this one for future assumption questions. Please advise. Thank you!

[Adam Tyson]

To negate a conditional statement, gen, you may need to do a little re-wording of the answer, but the final product will be to say that the necessary condition is not actually necessary. We often use the phrase "even if" to do that - the sufficient condition can occur even if the necessary condition does not. It's not necessary. That doesn't mean creating a conditional diagram where the necessary condition is negated, though, because that means that the necessary condition cannot occur, and that's too strong, and does not use logical opposition like we are supposed to do when using this technique.

In this case, you could negate answer E by saying the body’s immune system can recognize infectious bacteria even if there are not sufficient quantities of the bacteria to cause overt symptoms of disease. So, you don't have to have gotten obviously sick from the disease for the test to work. It could work even if you are exposed to a lower dose. That doesn't wreck this argument, but strengthens it!

In short, to negate a conditional statement, do not create a new conditional statement, but simply show that the necessary condition isn't really necessary.

[gen2871]

Yes sir!

[haileymarkt]

Hello!

When answering this question, I originally determined that B and C were my contenders. I did end up choosing C because I determined that if the proteins were present due to another bacteria, this would be an alternative cause (basically a false positive). However, I wasn't able to confidently rule out option B. Can you explain why B is incorrect?

My thought process was that if skin irritations are symptoms of TB patients, then they may already have rashes before the test. Thus this wouldn't necessarily mean that they don't have TB, just that the test doesn't have a good mechanism to prove that they have TB because they are already displaying symptoms. Is that on track?

Thanks for your help

[Adam Tyson]

It's not really about the symptoms of TB, haileymarkt, but about how patients would react to the test for TB. They author need not make any assumptions about how the disease itself manifests in patients. To prove that this answer is irrelevant, rather than a needed assumption, use the Negation Technique. What if localized skin irritations are NOT a symptom? Wouldn't the author react by saying "so what, who cares? All I carry about is how they react to this test." The argument remains untouched by this negation, and that means the author did not need to make that assumption in order for their argument to be valid.

[ericj_williams]

Can someone help me with my understanding.

I felt like C was the answer they wanted, but I thought about it and it doesn't really make sense.

So if the bacterium recognized is different, how does that tell me the person has TB?

Why would I test for something with a different substance? If the substance used in the test is different from that in body, how can the test tell me anything about what's in the body? They're different.

If the bacterium is exclusive to the test, how can I use that to tell me what's in the body?

[Adam Tyson]

"Exclusive to that bacterium" means that no other bacterium could cause that irritation, ericj_williams. In other words, you could not get that response (skin irritation) unless you had the tuberculosis bacterium in your system. What if that's not true? What if injecting someone with this protein might cause them to get skin irritation even if they don't have tuberculosis, perhaps because some other bacterium in that person's body reacts the same way to that protein? Then the test would be useless, because we wouldn't know if you have TB or the other thing that gives the same result.

To walk through the experiment, let's say someone the TB bacterium has protein X in it. You inject me with a little protein X to see if I react to it, which I do. Does this prove I have TB? Only if TB is the only thing that triggers that reaction. If not - if, say, streptococcus bacteria also reacts the same way to protein X, then maybe I have strep instead of TB? That's a problem, so the folks who believe the test is valid have to assume that problem does not exist. If you react to the protein, it can only be due to the TB bacterium being in your system. It's exclusive.