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Complete Question Explanation

Must Be True. The correct answer choice is (E)

Your task in this Must Be True question is to select the answer containing either a restatement of one
of the facts in the stimulus, or an inference permissible from a combination of those facts. The facts
contained in the stimulus are:

..... Fact: ..... birds and mammals can be infected with West Nile virus only through
..... ..... ..... mosquito bites

..... ..... ..... bird or mammal infected with West Nile ..... :arrow: ..... infected by mosquito

..... Fact: ..... mosquitoes become infected with the virus when they bite certain infected
..... ..... ..... birds or mammals

..... Fact: ..... virus originally detected in northern Africa and spread to North America in the
..... ..... ..... 1990s

..... Fact: ..... humans sometimes catch West Nile virus, but the virus never becomes
..... ..... ..... abundant enough in human blood to infect a mosquito

..... ..... ..... mosquito infected with West Nile ..... :arrow: infected by human blood

Your prephrase in this question is that you can join the first and last facts presented in the stimulus to
create an additive inference: since a human cannot infect a mosquito with West Nile, and birds and
mammals can be infected with West Nile only through mosquito bites, then the disease is not spread
by humans.

The incorrect answers will not contain information supported by the facts in the stimulus, either
because they are not mentioned by the facts, are not inferable from the facts, or are contradictory to

Answer choice (A): The facts states that humans “sometimes” catch West Nile. The word
“sometimes” is the logical opposite of “all the time,” and includes the possibility of something that
would be considered common. At very least, the definitive prediction that the virus will never be a
common disease among humans is not supported the facts.

Answer choice (B): While this answer seems reasonable from a common sense perspective, it
presents new information. The facts did not provide any information concerning where West Nile is
most common.

Answer choice (C): This choice is intended to trick people who confuse the fact that the virus does
not become abundant enough in human blood to infect a mosquito with the virus not being abundant
enough in human blood to cause symptoms of illness. The stimulus contains no information
regarding when or how people become symptomatic.

Answer choice (D): The stimulus stated only that West Nile was originally detected in northern
Africa. It did not provide any information regarding where more people are infected.

Answer choice (E): This is the correct answer choice. While the language “carried” is ambiguous,
with the language potentially referring to West Nile coming to North America but not spreading, the
context in the stimulus implies this choice refers to the disease spreading to North America in the
1990s. Since the combination of the first and last sentences tells you that West Nile cannot be spread
by humans to mosquitoes, birds or mammals, you can infer that West Nile was not carried to North
America via an infected person.
  • Posts: 38
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Hi. I got this question right through process of elimination, but I still want to know how e follows the stimulus. Pertaining to humans, the stimulus only says that humans can't infect mosquitos, not that they can't infect each other.
 Nikki Siclunov
PowerScore Staff
  • PowerScore Staff
  • Posts: 1364
  • Joined: Aug 02, 2011

Let's take a look at the first and last sentences:

1. Birds and mammals (incl. humans) can be infected with West Nile virus only through mosquito bites.
2. Humans cannot infect a mosquito.

So, how was the virus carried to North America in the 1990s? Clearly, it was not via an infected person, because such a person could not have infected any mosquitos (last sentence). And, since birds and mammals can only be infected through mosquito bites, it would have been impossible to spread the disease in North America if the carrier was a human.

Hope this makes sense! Let me know.


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