This stimulus presents an argument. The conclusion is "These facts in themselves demonstrate that treatments that raise the concentration of magnesium in the blood would provide an effective cure for the fatigue involved in the syndrome." The premises are that (1) chronic fatigue syndrome is invariably associated with lower than normal blood magnesium concentrations and (2) mal absorption of magnesium from the digestive tract to the blood is also often associated with some types of fatigue.
In other words, the stimulus establishes a correlation between blood magnesium levels and fatigue, but the conclusion assumes that the first thing causes the second. But what if it is the fatigue that is, for whatever reason, leading to lower levels of blood magnesium?
This is answer choice (B).
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