For years, doctors have been trying to find a cause for Chronic Fatigue Syndrome (CFS). The devastating illness tends to strike healthy and fairly active individuals with no other history of medical problems or conditions. Recently, doctors in California have found that over 70% of those individuals diagnosed with CFS are infected with a rare enterovirus that is almost never found in the healthy population. As a result, they believe that this enterovirus is the primary cause of many cases of CFS.

Which if the following is an assumption upon which the doctors’ theory depends?

A) There are not other viruses that may cause Chronic Fatigue Syndrome.
B) The rare enterovirus was present at the onset of these patients’ Chronic Fatigue Syndrome.
C) The laboratory technicians analyzing the presence of the enterovirus in the patients’ blood and tissues did not make some mistakes in their analysis.
D) The severity of the symptoms among the patients diagnosed with CFS does not vary widely.
E) The rare enterovirus is commonly seen in patients that are severely ill.

IMO :conclusion says that : enterovirus is the primary cause of many cases of CFS. not the only cause ...there may be other viruses

Premise itself says that : over 70% (((((not all))))of those individuals diagnosed with CFS are infected with a rare enterovirus that is almost never found in the healthy population

Hope it helps

None of the options seem convincing.
I was hoping to see something that might say something like..."the virus is produced as a result of the disease or genetic mutation due to illness" or else "the hormonal imbalance has been observed in patients, two years prior to the start of CFS"

Something that on negation disproves that the virus is the primary cause!
_________________

Desperately need 'KUDOS' !!

I chose the answer C for this.

I thought option B was wrong (The rare enterovirus was present at the onset of these patients’ Chronic Fatigue Syndrome.) because it seems irrelevant to be an assumption to the doctors theory.

Can someone please explain why B is the right answer.

Hi pra1785

If you have narrowed down to option "B" & "C" then you can use the negation technique to arrive at the correct answer.
The conclusion of the argument is "enterovirus is the primary cause of many cases of CFS".

Negating option B we get: The rare enterovirus was NOT present at the onset of these patients’ Chronic Fatigue Syndrome.
if the virus was not present then it cannot be the cause of CFS. Clearly our conclusion is shattered. Hence option B is the correct answer.

Negating option C we get: The laboratory technicians analyzing the presence of the enterovirus in the patients’ blood and tissues did not make made some mistakes in their analysis.
the tehcnicians made some mistake in analysis. so let's assume that instead of virus being 95% responsible for illness is now found to be 60% responsible for illness and rest 40% could be some other factor. But this finding does not disapprove the fact that the virus is not the primary cause of illness. Hence this option has no impact on the conclusion.

VERITAS PREP OFFICIAL SOLUTION:

In this Assumption question, the original argument contains a classic logical fallacy - correlation versus causation. Just because one thing is correlated with another, it does not mean that one is causing the other. Here evidence is given that 70% of CFS patients tested positive for a rare enterovirus, so indeed a clear correlation exists between the virus and CFS. However, the scientists’ conclusion is that the rare enterovirus must be causing many of the cases of CFS and there is not enough evidence given for that. Clearly the following scenario is possible: people first get sick with CFS and the cause is not the rare enterovirus but something else; their immune systems are decimated by CFS and they are then infected by a this rare enterovirus because of their CFS and not vice versa. If some information was given that linked the virus to the onset of the disease, this particular flaw would be corrected and the argument would be better (but still have other problems). Answer choice (B) does that perfectly by eliminating the scenario described above and strengthening the argument.

For (A) it is not important to know if other viruses do or do not cause CFS. The scope of the conclusion is just that the enterovirus is the cause of many cases, so the argument is not affected by whether some cases may or may not be caused by other viruses. For (C), it would clearly matter if the technicians made mistakes on a large portion of the data but the word “some” here is problematic. As “some” can mean anything from 1 to all, you are not sure whether this is really a necessary assumption. If the technicians made only one mistake in all the data analysis, then that would clearly not be a problem in this argument. (D) is perhaps the easiest to eliminate as the severity of symptoms does not relate to the problem of correlation/causation discussed above. For (E), it is not important what role the rare enterovirus might play in other severe illnesses (this is outside the scope of this argument). However, if anything, (E) would weaken this argument as it might suggest the scenario described above: that the enterovirus is an opportunistic virus that appears after people get very sick and thus might not be the cause of CFS but rather a result. The correct answer is (B).
_________________

आत्मनॊ मोक्षार्थम् जगद्धिताय च

Resource: GMATPrep RCs With Solution