We have a CLASSIC LSAT argument in this stimulus: mistaking correlation for causation. The stimulus tells us that
"Chronic fatigue syndrome is invariably associated with lower-than-normal concentrations of magnesium in the blood." Basically, people with chronic fatigue syndrome have lower-than-normal concentrations of magnesium. The second sentence is supposed to serve as additonal evidence to reinforce this connection. Ok. That's great, you think.
We then come to the conclusion that all of the above-information supports the belief of the author that "treatments that raise the concentration of magnesium in the blood would provide an effective cure for the fatigue involved in the syndrome." Two things should jump at you within this conclusion. First, the author has now assumed correlation for causation i.e. the low magnesium leads to chronic fatigue syndrome and the way to cure this is by increasing the level of magnesium. This is where the argument goes terribly wrong and you should be able to take advantage of the authors flawed logic. You should ask yourself, wait, how do we know the low magnesium levels cause chronic fatigue syndrome? What if the relationship is reversed?? This is exactly what (B) gets at, and if correct, really hurts the conclusion. Also, notice the scope shift. author starts talking about treatments. that should have raised a red flag.
Now for the inccorect choices:
(A) this does not need to be established. Focus on disconnecting the premise(s) and conclusion. Ask yourself, if this were established, would it really hurt the argument?
(C)this is out of scope. we are only concerned with people who have chronic fatigue syndrome. that is what the conclusions focuses on.
(D)this is irrelevent. we do not need to know the exact concentration. it would not hurt the argument.
(E)knowing what methods are "most" effective is irrelevant to the conclusion. it misses the point of the argument, which is this idea misinterpreting correlation for causation. this is what we want to weaken, but (E) does not help.
Btw, I know the first two paragraphs are lengthy, but that is the thought process you should have when attacking the arguments.
I hope this was helpful! Let me know if you have any other questions.