Doctors sometimes have difficulty diagnosing viral pneumonia because

OFFICIAL EXPLANATION

Here’s How to Crack It

Go through your checklist: Do you see any suspicious pronouns, misplaced modifiers, or unparallel constructions? Good. There aren’t any. Do you see any comparison words? Yes, the sentence uses “are” and “similar to.” Let’s see exactly what is being compared. The symptoms of one illness are being compared directly to … another illness. Aha! This is a parallel comparison error. To make this sentence correct, we need to compare the “symptoms” of one illness to the “symptoms” of the other, and the way GMAC would prefer that we do it is by using a replacement pronoun.

If we look at the answer choices, we can eliminate choices A and E because neither makes any attempt to compare symptoms to symptoms. Choice B looks promising because it uses the replacement pronoun “that”; however, “symptoms” is plural and therefore can’t be replaced by the singular “that.” Choice D seems promising because it looks like it’s trying to compare symptoms to symptoms— but if you look more closely, you’ll notice that the last word of choice D is “symptom,” which is singular. The correct answer is choice C.