A common genetic mutation that lowers levels of the enzyme cathepsin C severely reduces a person's ability to ward of...

Serra on May 3 at 07:40PM

E vs. D

Could someone please explain why E is correct and D is not?

3 Replies

Ravi on May 5 at 07:59PM

@msaber,

Happy to help. This is a strengthen with a necessary premise question.
We're looking for an answer choice that must be true in order for the
argument to stand a chance. We can use the negation test with the
answer choices to identify the correct answer. If the negated answer
choice wrecks the argument, then we know that the original answer
choice must be true in order for the argument to have a chance, making
it the correct answer.

You asked about (E) and (D), so let's take a look.

(D) says, "Persons who do not have the genetic mutation that lowers
levels of cathepsin C do not get gum disease."

(D)'s negation is, "Persons who do not have the genetic mutation that
lowers level of cathepsin C can get gum disease."

(D) is actually a sufficient premise, so it would make the argument
valid. However, it isn't required, so it's not a necessary premise. We
know this because in looking at the negation, it does not
substantially undermine the conclusion of the stimulus (it doesn't
wreck the argument). Even if these people can still get gum disease,
restoring the levels of cathepsin C to normal ranges may still fix the
gum disease they have. Thus, (D) is out.

(E) says, "A person whose cathepsin C level has been restored to
normal will not suffer from periodontitis."

(E)'s negation is, "A person whose cathepsin C level has been restored
to normal will suffer from periodontitis."

(E)'s negation makes the argument fall apart because it totally wrecks
the conclusion of the argument. From this, we know that (E) is
required for the argument, so it's a necessary premise and is the
correct answer choice.

Does this make sense? Let us know if you have any more questions!

Serra on May 6 at 02:12AM

This is so awesome thank you! These question types are my biggest weakness because I didn't understand how to do the negation test so it is really helpful to see it broken down so clearly. (D) was also one of those tempting answer choices and they totally got me! Now that I have a better understanding of how to apply the negation test (E) is the obvious choice. Aaaaaand now its time to review that lecture :). Thanks again!

Ravi on May 6 at 05:55PM

@msaber, happy to see this helped! :) Keep us posted with any additional questions you may have!