The author thinks that restoring this enzyme to normal levels represents one way to cure gum disease, but there might be other cures as well. In other words, the existence of another cure for gum disease does not invalidate the author’s proposed cure.

The argument assumes that lowered cathepsin C is the only cause of gum disease, not that the genetic mutation is the only cause of lowered cathepsin C. There could be many causes of this lowered enzyme, but as long as researchers can consistently bring it back up to normal levels, then those other causes don’t matter.

The conclusion is qualified by the statement, “Once [researchers restore the enzyme to normal levels].” It doesn’t matter if they actually accomplish this task—only that gum disease would be eliminated if they do.

If this answer choice instead said, “Persons who do not have lowered cathepsin C levels do not get gum disease,” it would be necessary. As it stands, however, the argument is fine if people develop gum disease without the genetic mutation. They could have lowered cathepsin C from some other source.

This references one of the author’s assumptions—that the lowered enzyme is the only way to get gum disease. If people could get gum disease in other ways despite researchers fixing the lowered enzyme, then gum disease would not be successfully eliminated, and the conclusion would fall apart.