June 2012 LSAT Section 2 Question 5

Bacteria that benefit human beings when they are present in the body are called commensals. The bacterium Helicobacte...

MACZ on December 23, 2019

I saw the last post but I was hoping for some clarification as to why C would be the right answer. The way I am reading it, I see it strengthening the arguement not weakening it.

Since "Bacteria that benefit human beings when they are present in the body are called commensals," then wouldn't C strengthen the arguement by saying that M. tuberculosis doesn't provide a benefit for people that harbor it. The conclusion is that it isn't a commensal. If the author argued that it was a commensal then this would weaken it but he doesn't. If I were to weaken the arguement wouldn't I want to prove that it is a commensal? Let me know if I am way off base here, thanks!

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Annie on December 24, 2019

Hi @MACZ,

This question asks you to find the answer choice which undermines the argument's reasoning. Here's a breakdown of the argument:

Premise: Good bacteria are called commensals.
Premise: The bacterium HP is involved in stomach ulcers but those only happen in 10% of people with HP bacteria.
Premise: HP bacteria also allegedly strengthens immune response, so it's a commensal.
Conclusion: This is wrong because only about 10% of people with MT get tuberculosis, but that doesn't make it a commensal.

As you can see the argument is trying to draw a parallel between HP and MT so show that the classification of HP as a commensal is wrong. You're trying to undermine the argument, and a good way to do it is to undermine this parallel.

Answer (C) is correct because it does just that. Premise 3 tells us that HP helps strengthen immune response, while this answer tells us that MT does not have this affect. This difference drives a wedge in the parallel, and thus weakens the argument.