For years, doctors have been trying to find a cause for Chronic

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. Now consider, B. The rare enterovirus was present at the onset of these patients’ Chronic Fatigue Syndrome.

Since enterovirus almost never found in the healthy population and also the primary cause of many cases of CFS, we may conclude, enterovirus was present at the onset of these patients’ Chronic Fatigue Syndrome.

Answer: B.

Why not option A as if u negate it there are other viruses that caused CFS as conclusion tells EVIRUS is the MAIN reason for CFS.

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Because the text specifies that the doctors believe it to be the primary cause of the illness, which leaves space for different secondary causes. By negating (A) you do not negate the assumption made by the doctors.

Argument Breakdown:
Doctors observed that 70% of people with CFS are infected with a rare enterovirus.
Based on this correlation, doctors hypothesize that this virus is the primary cause of many CFS cases.
An assumption bridges the gap between the evidence (enterovirus found in most CFS patients) and the conclusion (the virus is the primary cause).

Let's evaluate each option:
A. There are not other viruses that may cause Chronic Fatigue Syndrome.
If there are other viruses that cause CFS, the rare enterovirus might not be the primary cause. The doctors' theory depends on the idea that no other viruses are likely culprits.
This is a strong contender since it directly supports the theory that this virus, and not another, is the primary cause of CFS.

B. The rare entervirus was present at the onset of these patients’ Chronic Fatigue Syndrome.
For the enterovirus to be the cause, it must have been present at the beginning of the illness, not after symptoms appeared. If the virus appeared later, it might be a consequence rather than a cause of the illness.
This is also a necessary assumption because without the virus being present at the onset, the theory that it caused the syndrome wouldn’t hold.

C. The laboratory technicians analyzing the presence of the enterovirus in the patients’ blood and tissues did not make some mistakes in their analysis.
If the lab made mistakes, then the data the doctors are relying on could be faulty, weakening the theory. However, the argument is more about the relationship between the enterovirus and CFS, not about procedural accuracy.
This is not as directly tied to the theory. While errors in the lab could undermine confidence, this isn't a fundamental assumption of the argument.

D. The severity of the symptoms among the patients diagnosed with CFS does not vary widely.
The severity of symptoms might vary, but this doesn't necessarily affect whether the enterovirus causes CFS. The focus of the argument is on the cause of the syndrome, not its severity across patients.
This is irrelevant to the argument's conclusion about the cause.

E. The rare enterovirus is commonly seen in patients that are severely ill.
Whether the virus is seen in other severely ill patients doesn't necessarily support or weaken the doctors' claim that it's a cause of CFS. The argument is specific to CFS, so the virus's presence in other illnesses is not important.
This is irrelevant to the argument.

Conclusion:
A and B both seem like strong contenders, but B is more directly necessary for the conclusion. If the virus wasn’t present at the onset of CFS, the argument falls apart, making B the assumption upon which the doctors' theory depends.
Correct answer: B.

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).