E vs. D

@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!

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!

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