It's B

IMO B

This suggests that the new drug acts only by inhibiting the bacteria from coming into the blood stream and not by any other method (say by eliminating the bacteria present in the blood stream etc....).

Thus it weakens the conclusion that this new drug will prove to be toxic for bacteria!

This is a cause-effect type of question. Always start with the effect that they are trying to explain. The effect will be a result or a behavior that is observed and is not something to be questioned.

In this case, you have the observed effect of "when mice are given both morphine and the new drug naltrexone, blood poisoning is much less frequent..." So we are trying to explain why the blood poisoning is much less frequent.

The cause is something that you can argue about because it is debatable and is not an observed fact. In this case the proposed cause is "These results provide support for researchers' prediction that naltrexone will turn out to be toxic to certain types of bacteria." So the proposed cause sort of hidden in the statement is that naltrexone is toxic to certain types of bacteria -- specifically the types that normally get into the bloodstream when morphine is taken.

Now on the GMAT THE classic way to weaken cause and effect is with an alternate cause. There are other ways to weaken and the LSAT relies on several other ways, but the GMAT usually relies on offering another cause that could also explain the effect.

Answer choice B gives us such a cause. B states, "Naltrexone inhibits morphine from triggering the migration of intestinal bacteria into the bloodstream." If this is true (and you are supposed to evaluate each answer choice by pretending that it is true) then there is no need to for naltrexone to kill the bacteria in order to keep it from the bloodstream. It could also simply help keep the bacteria in the intestine where it belongs.

Choice C says "Mice that have been given naltrexone but not morphine have no greater risk of developing blood poisoning than do mice that have not been given either substance." This does not point to any sort of conclusion. The fact that there is no increase in blood poisoning when only naltrexone is used does not mean that it does or does not kill bacteria. It could be that it has no effect at all and that is way taking it alone has impact. This choice does not weaken the conclusion that naltrexone kills bacteria.

Hope it helps!

Hi David,

I think C is irrelevant and AT MOST Strengthens the conclusion. Lets say when neither M nor N is given level of blood poisoning is 8(on a scale of 10) and when only N is given it is not greater than 8 i.e. it could be 8 or may be 6. When i remains 8 it does not have any effect and when it goes to 6 then it may be the case that N kills bacteria and blood poisoning is reduced. However, our job is to find out other reason and hence weaken the given reason for reduction of blood poisoning.

Got a PM on this one...

Vikram - I certainly agree that C is not the answer. In my post above I stated that choice C does not weaken the conclusion. Choice B is the one that weakens the cause and effect here.

So yes, I agree with you on this one!