If medical researchers are correct, then the human microbiome, made up of the microorganisms in our body, may hold the cure to diseases that have long plagued humanity, amounting to a major oversight in Western medicine that has, until recently, all but ignored any such role of the microbiome.
(A) humanity, amounting to a major oversight in Western medicine that has, until recently, all but
(B) humanity, a discovery that amounts to a major oversight in Western medicine, which, until recently, had all but
(C) humanity, a discovery amounting to a major oversight by Western medicine: until recently, Western medicine all but
(D) humanity, amounting to a major oversight made by Western medicine, one that, until recently, all but
(E) humanity, which amounts to a major oversight in Western medicine and until recently it all but
The Summative Modifier
A very subtle point that will come up on the harder GMAT SC questions is the idea of a summative modifier: a word that “encapsulates” the action of the preceding clause. To illustrate: the original sentence states that “microbiome…diseases…humanity, amounting…”. In this case, what does “amounting” refer to? Whichever of the three you argue for—assuming you argue for any—none amount to a major oversight in Western medicine. “The microbiome amounts to a major oversight” is odd, though the least odd of the three.
In order to clearly state what is doing the “amounting”, we use a word that “encapsulates” or captures the preceding phrase. This word is known as a summative modifier. In other words, it sums up what is being said. The good news here is you do not need to come up with the summative modifier itself, but you will have to find an answer choice that uses one.
Side note: the GMAT will never have multiple possible summative modifiers, asking you to pick the word that best encapsulates the preceding clause.
In this question, “a discovery” is a perfect summative modifier since it captures the idea of the preceding clause: the micribiome may hold cures to disease, a discovery…”
Just like that we can eliminate (A), (D) and (E).
The difference between (B) and (C) is somewhat similar. But instead of using another summative modifier, which is stylistically off, the correct answer, (C), uses a colon and cleans up the error by clearly stating who has been doing the ignoring: Western medicine.
As for (B), the “which” refers to the phrase “a major oversight”. It is not the “major oversight” that has until recently been ignored.
Finally, the “until recently + verb tense” is one of those faux errors that test takers can get hung up. There is no hard and fast rule here. Either works.
What the GMAT does want you to know is how to correctly modify clauses. And knowing how a summative modifier works will go a long way.
Oh, if you missed it, the answer to the question above is (C).