Self-study
This necessary assumption question discusses the treatment of Chronic Fatigue Syndrome (CFS) with a newly developed drug. CFS is associated with three different symptoms, and we don’t know if these symptoms are the effects of only one virus or of multiple different ones. Tests of the new drug indicate that this drug lessens the severity of all three CFS symptoms. The stimulus takes this to provide evidence to the effect that CFS probably is caused by one single virus, not by multiple different ones.
Pre-phrase / anticipation: We need an assumption to the effect of ‘If a single treatment lessens all of a given syndrome’s symptoms, then it is more likely for this syndrome to be caused by a single virus than by multiple ones.’
The pertinent answer choices are (B) and (D). (B) states: “It is more likely that the new drug counteracts one virus than that it counteracts several viruses.” This matches the consequent in the anticipated assumption but leaves out its antecedent. (B) thus does not make the argument valid and would fall short of being a sufficient assumption. But is (B) necessary? If negated, (B) would indicate that it would be equally likely or even more likely that the new drug affected several viruses. This is not at all what the author is trying to argue and thus would seem to rob their conclusion of any support.
(D) states: “Most syndromes that are characterized by related symptoms are each caused by a single viral infection.” This in itself might be right, and arguably (D) would be a good strengthen answer choice. (D) gets at the conclusion and points out parallel cases where similar correlations have been observed as well. A number of things seem off though: (1) Do we know that the alleviated symptoms in fact are ‘related,’ as this answer choice suggests? We certainly know that they all are effects of one or more causes, but does that also render these effects related to one another? (2) The conclusion in the stimulus takes the results of the experiments with the new drug to provide evidence to the effect that CFS has a single cause, but (D) does not contain a connection to these experiments. Instead, (D) is just making a general claim that arguably strengthens the conclusion in isolation but that does not also connect it to the other parts of the argument.
As an NA answer choice, (B) thus seems better than (D). (B) is essentially saying: In probabilistic terms, the new drug’s acting on three different effects indicates that these three effects likely have a single cause rather than three different ones. By contrast, had the drug only affected two of CFS’s three symptoms, it would have been likely that there are at least two causes for CFS, one virus that triggers two of its symptoms and another virus that triggers the third one. (B) is thus hinting at a sort of appeal to simplicity behind the author's reasoning. The author seems to assume: If two different hypotheses about the causal relationships behind a given correlation are possible, the simpler hypothesis is more likely correct.
