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

Ravi on December 16, 2018

@letc,

Great question. Before reading the question stem or looking at the answers, it's important that we have a strong grasp of what's going on in the stimulus.

The author's conclusion is that scientists considering H. pylori to be a commensal is misguided. The author supports this assertion by using an argument by analogy with the M. tuberculosis. Since we're looking to weaken the argument, what we need to do is weaken the analogy the author makes between H. pylori and M. tuberculosis.

The author mentions that H. pylori allegedly strengthens the immune system. One way we could destroy the analogy that the author uses would be to add something that shows that M. tuberculosis either 1) hurts people or 2) does not provide a benefit to those who harbor it. This would hurt the analogy because it would weaken the similarity that the author is attempting to show between the two bacteria.

Answer (A) shows how the bacteria are similar and does not weaken the argument.

(B) mentions the time of cases of tuberculosis vs. ulcers, but this is not relevant to the discussion.

(C) This answer would weaken the argument because it shows a key difference between H. pylori and M. tuberculosis. We know that H. pylori allegedly strengthens immune response. With answer (C), we know that M. tuberculosis provides no benefit to people who harbor it. If this is true, then the author's conclusion—that the scientists are misguided in considering H. pylori to be a commensal—is strongly weakened. This is the correct answer.

(D) says there are more people with M. tuberculosis than H. pylori. So what? What if it's just one more person? This does nothing to weaken the argument. If we wanted it to weaken the argument.

(E) says there are more people with H. pylori than M. tuberculosis. Again, why does this matter? It could mean there is only one more person with H. pylori than M. tuberculosis. The number of people with these bacteria has nothing to do with our argument.

I see you chose (E). The problem with the reasoning you provided is that in order for it to work, you have to make an assumption that there is a connection between the number of people harboring a bacteria and its ability to act as a commensal. This may or may not be true, but it's an additional assumption that would need to be made for this answer choice. Additionally, this reasoning is also failing to draw a meaningful distinction between the two bacteria, as answer choice (C) does.

Hope this helps. Let me know if you'd like further clarification!