Hey folks,

Hopefully I can help you out with this one:

A. We have a bonkers number of commas in this sentence, which is often an indicator that something could be said more clearly. However, we also have a bonkers number of commas in the other answer options, so that doesn't help us much. We should recognize that a bunch of the stuff set off by commas is extra information: "according to the diagnosis" provides more information about "the most detrimental", and "or high blood pressure" explains what "hypertension" is (in other words "hypertension" = a fancy word for "high blood pressure"). Cutting these and some extra descriptive fluff out of the sentence gives "The doctors said that the most detrimental [factor] was hypertension and that luckily this factor is the most controllable element." This makes sense. Keep for now.
B. We notice that the singular "was" from A switches to the plural "were" in this answer choice, so we likely have an agreement error. What noun does our verb refer to? "The most detrimental [out of the several factors that caused the lady's stroke]". This is a confusing construction - likely the question writer hopes that test takers will see "factors" and think "plural". However, the subject is the ONE most detrimental factor out of all of these factors. This is singular, so we need a singular verb. "Were" is plural, so eliminate.
C. This answer changes the meaning of the original sentence. Rather than explaining that "hypertension" = "high blood pressure", the sentence now says that they are two different things and that either one of them could be the most detrimental factor in the lady's stroke. If you know much about biology, you know that this doesn't make any sense. But you don't know anything about biology, you can still answer this question! On the GMAT, we always want to match the meaning of the original sentence, so we can rule out this answer choice. We also notice that this answer choices gives us "these factors is the most controllable element", where we pair a plural noun ("factors") with a singular verb ("is"). Eliminate.
D. Again, this answer choice changes the meaning: it says the most detrimental factor was "hypertension", but that "high blood pressure" is controllable. This doesn't make sense. 1) This says that the two things aren't the same. 2) If they aren't the same, why would it be "lucky" that high blood pressure is controllable? Hypertension is her problem! So the meaning is both different and illogical. Eliminate.
E. The main issue here is that "diagnosis" is a singular noun, while "have shown" is a plural verb. Eliminate.