­Among people in Tiravia who had a fatal car accident while driving

­Among people in Tiravia who had a fatal car accident while driving, 5 percent of those under 50 years old were found to have prosinopsis, a currently untreatable eye disease that causes a gradual deterioration in peripheral vision. Yet, according to medical records, only 3 percent ofTiravians under 50 have been diagnosed with prosinopsis. Therefore, when a Tiravian driver under 50 has prosinopsis, the disease significantly increases the likelihood that the driver will have a fatal car accident.

The passage presents an argument with the following conclusion:

when a Tiravian driver under 50 has prosinopsis, the disease significantly increases the likelihood that the driver will have a fatal car accident

The reasoning of the argument is that, since a greater percentage of people under 50 involved in accidents, 5 percent, than of people under 50 in the general population, 3 percent, have been found to have prosinopsis, it must be that the disease causes people to have accidents.

Which of the following, if true, most seriously weakens the argument?

The correct answer will somehow indicate that, even though it's true that a greater percentage of people involved in accidents than of people in the general population have been found to have prosinopsis, it may not be the case that the disease causes people to have accidents.

A. People who have prosinopsis in its later stages are generally unable to drive because of the marked deterioration in their peripheral vision caused by the disease.

If anything, this choice strengthens, rather than weakens, the argument.

After all, if what this choice says is true, then it's clearly the case that the deterioration associated with prosinopsis can render people unable to drive effectively. So, it could be that, even when the disease is not "in its later stages," people who are experiencing it lose some ability to drive safely and, as a result, get into accidents.

Eliminate.

B. There are reliable tests that will allow prosinopsis to be diagnosed even in its earliest stages.

We already know from the passage that prosinopsis can be diagnosed. So, all this choice adds is that it can be diagnosed in its earliest stages.

The fact that it can be diagnosed in its earliest stages doesn't mean that it doesn't cause people to have accidents. After all, regardless of when it can be diagnosed, at some point in its progression, it could cause people to have accidents, and the evidence provided seems to indicate that it does.

Eliminate.

C. When prosinopsis is in its early stages, its effects are detectable only by means of laboratory tests, which few Tiravians under 50 undergo.

This has to be one of the least obvious Critical Reasoning correct answers I've seen. To see why this choice is correct, we have to make the following connections.

The accidents discussed in the passage are "fatal car accidents." So, using our common knowledge of the world, we can decide that it's reasonable that these accidents will be investigated.

In that case, "laboratory tests" may be done, in which case the driver may be "found" to have been experiencing prosinopsis. Notice that the passage doesn't say that these drivers had been "diagnosed" with prosinopsis. So, they were somehow "found" to have been experiencing prosinopsis after they crashed, and a logical way that finding would occur would be through laboratory testing.

So, in that case, we have an alternative explanation for the slightly higher incidence of prosinopsis in people who have fatal car accidents. That explanation is that, it may not be that the deterioration associated with prosinopsis causes accidents but, rather, that being in an accident causes a person to be more likely to be found to have been experiencing prosinopsis. After all, according to this choice, "few Tiravians under 50 undergo" testing for prosinopsis. So, it makes sense that laboratory testing done after a crash would reveal prosinopsis in a greater percentage of people than are normally diagnosed with the disease.

So, this choice weakens the case for the conclusion by indicating that something other than prosinopsis causing accidents could be what's going on.

Keep.

D. Although a large majority of Tiravians who have been diagnosed with prosinopsis are over 50, many are in their 30s and 40s.

This choice has no effect on the strength of the argument.

For one thing, we already know from the passage that 3 percent of people under 50 are diagnosed with prosinopsis.

Furthermore, the fact that people in their 30s and 40s specifically are diagnosed with prosinopsis doesn't indicate in any way that prosinopsis doesn't cause people to have accidents because there's nothing specific to people of those ages that's related to crashing cars.

In fact, if anything, this choice strengthens the argument by serving to rule out that people of a certain age group that would be more likely to get into accidents, such as drivers under 18, are most of the people diagnosed with prosinopsis. So, this choice helps to confirm that the deterioration associated with prosinopsis, and not the way people in a particular age group drive, is responsible for the accidents.

Eliminate.

E. Prosinopsis causes no significant deterioration in a person’s vision other than the deterioration in peripheral vision.

This choice could appear to weaken the argument by indicating that maybe prosinopsis doesn't do enough to vision to cause accidents. In other words, we might be tempted to think that, if prosinopsis causes no significant deterioration in a person’s vision other than the deterioration in peripheral vision, people who are experiencing prosinopsis should be able to drive safely and that, therefore, the crashes are not due to people's experiencing prosinopsis.

At the same time, the truth is that this choice doesn't change the fact that prosinopsis does affect vision, and thus could affect driving ability.

Also, this choice doesn't weaken the support provided by the fact that a greater percentage of people inolved in accidents than of people in the general population have been found to have prosinopsis. Regardless of how prosinopsis affects vision, that information supports the conclusion.

Eliminate.

Correct answer: C­