Q6 - One test to determine whether a person

[Mab6q]

Can anyone please shed some light on this question. Why would B not work? It seems to me that if I negated it, it would kill the argument. Thank you.

[yiwoo0216]

I believe B only states that it is a characteristic of tuberculosis regardless of whether the test was applied. If the test isn't applied and there is skin irritation, then the test itself wouldn't be very robust because one of the key indicators is already present.

If i'm not mistaken, i believe this would weaken the argument.

[rinagoldfield]

This is a necessary assumption question, so let’s start with the argument core.

Premise: People with TB respond to the injection with a skin irritation

Conclusion: People who respond to the injection in this way must have TB

Next we want to ID the logic gap here. The physicians assume that nothing other than TB could cause this particular response to the injection (the skin irritation). After all, if, say, a common cold could also cause the irritation, then the irritation would not necessarily signify TB. It could also signify a cold.

Ok, now to the answer choices.

(A) is way too broad. ALL proteins? We’re concerned with TB proteins.

(B) is irrelevant. We’re looking to connect the TB TEST to the skin irritation, not TB to the skin irritation.

(C) is EXACTLY what we’re looking for. We’re looking to eliminate the possibility that something else (like a common cold) could cause the skin irritation. (C) says precisely this"”the skin irritation is "exclusive" to the bacterium.

(D) is irrelevant. We want to know if the injection is a good diagnostic tool, not if it can cause TB.

(E) is also irrelevant.

[WaltGrace1983]

I'd lack to add a little bit here because when I first saw this question I had to think too much

(A) The reason why I eliminated this was because it actually goes against the premise. The premise states "the person's immune system recognizes certain proteins present..." Thus, we don't need the body to recognize all proteins, just the ones that matter.

(B) Like someone else mentioned, (B) is no good because the skin irritations don't have to be a "characteristic symptom." They don't have to be a symptom at all of TB. They only have to be a symptom of the injection. For example, losing one's hair is not a symptom of cancer (that I know of, at least). However, losing one's hair is a symptom of chemotherapy.

(C) I don't know if it is because I am tired today or what but when I was reviewing this question, this wording confused me. I want to break this down just to force myself to do it again. It is basically saying, "Only the TB's proteins have the ability to trigger the skin irritation." In other words, "Skin irritation → TB." We have to assume this because it shields us from the possibility that someone else is causing the skin irritation: maybe someone's skin is really sensitive that day because it is dry outside or maybe one is sick and so the immune system is weak, etc.

Hope that adds even just a little bit.

[robinzhang7]

Hey all, I completely understand why C is the correct answer. However, I am still confused on why (B) is wrong...

My line of thinking is this: Regardless of whether localized skin irritations are characteristic of "tuberculosis as a disease" or through "testing," the point is that skin irritation occurs.

When I negate (B) it turns out to be: Localized skin irritations are NOT symptoms of tuberculosis. The argument is hurt because the conclusion states:

Skin irritation --> infected with TB

Thus, I am wondering if the reason why (B) is wrong because "symptoms of TB" don't equate to being "infected with TB?????"

Am I making sense?

[rinagoldfield]

Thanks for your post, robinzhang7.

I think you make a good point. I will go against my earlier argument against (B) and make a different argument. I think (B) is wrong because it is a premise booster. The argument states that “This recognition [of the TB proteins] … results in a skin irritation.” Since we already know that skin irritation is a symptom of TB injection, (B) can’t be an assumption.

[donghai819]

This argument is very interesting in terms of the way it proceeds.

In the first line the author sets up the goal of the test-to determine the infection of tuberculosis. Then, the second sentence is interesting in that he turns to describe a theory about what is going to happen next (clue: "a" bacterium instead of "the" bacterium). We can symbolize it as "Infected --> immune system recognizes the proteins and attacks it". In the third sentence, what he says is, "indeed, the general idea I described in the second sentence does happen in this case AND it also results in a skin irritation." Thus we can further add "immune system recognizes the proteins and attacks it --> skin irritation ". Therefore, before proceeding to the conclusion, we would have "infected --> immune system recognizes the proteins and attacks it--> skin irritation". The authors concludes that, "anyone reacts in this manner to an injection with the tuberculosis proteins has been infected with tuberculosis". What he says is essentially an illegal negation of the conditional statement we have reached by the last sentence before the conclusion; that is, what he says is, "if the skin irritation is resulted in such a procedure, it is because of the infection.

Now let's go to the answer choices. We'd agree that there are actually only two candidates, B and C.

First and foremost, let me explain why C is right. C is right because it excludes the possibility that the skin irritation is caused by other things (maybe needles could cause skin irritation, among many other potential causes). Essentially, I believe, C actually transforms the conditional statement (infected --> immune system recognizes the proteins and attacks it--> skin irritation) into a bio-conditional; that is, if skin irritation happens, then the infection follows, given that it happens in "this manner"(line 10).

Second, let me explain why B is incorrect explicitly and concretely.
The logical negation of B would be: Localized skin irritations are NOT a characteristic symptom of tuberculosis in less a half people. What does this mean? It means that if we have a 100 people as a sample, for at most 49 of them localized skin irritations are NOT a characteristic symptom of tuberculosis; for at least 0 of them localized skin irritations are NOT a characteristic S of T. See, the latter scenario DOES NOT destroy the argument because if it is not characteristic symptom for nobody, it is characteristic for everyone, which is 100% consistent with the argument. Therefore, B is wrong. B might be a better choice if it is written in this way: at least for some people, localized skin irritations are a characteristic symptom of tuberculosis. But, it is still not as good as C, which is 100% solid.

A complicated but juicy question. And this one is an unicorn, because only very few necessary assumption questions heavily hinge on conditional logic. If I am correct, this form is the only way to test conditional logical under necessary assumption questions.

[MMeissner947]

To clear up why B is wrong, just check the negation test. Localized skin irritations are not a characteristic symptom of tuberculosis in most people. This doesnt get at the injection first of all, so we dont care whether its characteristic of just having tuberculosis, because the conclusion is about reacting with irritation to an injection, not reacting to tuberculosis infection. (I THINK should be prefaced before all of these sentences )

[MahmoudA830]

I'm still really confused about why (C) is a required assumption here. That just doesn't make any sense to me. If you injected someone with TB proteins, what does it matter if anything else can cause skin irritation? A million different things could cause that reaction and that still wouldn't sound like an issue to me. Heck we could even have a couple million more causes just for good measure and we'd still be fine. If ONLY ONE of those several million things were to be injected under the skin and cause a reaction, then we know we have a positive test for the specific set of proteins that were injected. Either the body recognizes those proteins (positive result) or it does not (negative result). Why would we care about what else could have caused it? I just don't at all see why the TB proteins need to exclusively cause the skin reaction.

[Laura Damone]

Sorry for the delayed response here!

The physician's reasoning is that "anyone who reacts in this manner to the injection with the TB proteins has been infected with TB."

That's a conditional statement:

React --> Infected

In order for that to be true, we need to assume that there are no false positives. In other words, there can't be anyone who does react but who is not infected.

That's where C comes in. When it tells us that the reaction is exclusive to TB proteins, it rules out the possibility of false positives. There can't be any other bacterium present in the body of a reactor that would trigger the reaction other than TB.

Hope this helps!