Antigenic shift refers to the combination of two different

I am responding to a pm from Archit143. First of all, Archit143 --- are you aware how many spelling mistakes you make in your post? This is not counting your own idiosyncratic abbreviations like "IIism." If you want to reach GMAT SC standards, I would highly recommend: make it your ongoing habit, in everything you say, and in everything your write, to conform at all times to the very highest principles of the English language. It believe it is a grave mistake to think of the GMAT as a hoop consisting of arbitrary standards, and once you jump through it you will never need to concern yourself with those standards again. Rather, the standards of the GMAT are the standard of business school and they are the standards of success in the corporate world. Do you want to succeed in that world? Then make excellence in all things a habit. ----- A fellow GC user right now might, in 15 years, be the person who is a position to make a lucrative deal with your future company --- suppose that person remembers your spelling mistakes here and thereby has a lowered opinion of you, and suppose that affects the deal. Would the 10 seconds it takes now to correct your spelling seem worth it? In life, we only get one chance to make a first impression on anyone; moreover, in our modern hyper-connected world, we never know when something we put out into cyberspace might be making that first impression in our name. All psychological studies show that first impressions are massively influential --- if someone forms a bad first impression of you, you practically have to walk on water to change that. You can't afford ever to put less than your very best out into the world. Does that make sense?

OK, in this sentence --- I think your discussion is missing the main points.
Antigenic shift refers to the combination of two different strains of influenza; in contrast, antigenic drift refers to the natural mutation of a single strain of influenza.
(A) influenza; in contrast, antigenic drift refers to the natural mutation of a single strain of influenza
(B) influenza, different than the natural mutation of a single strain, known as antigenic drift
(C) influenza, in contrast to the natural mutation of a single strain, known as antigenic drift
(D) influenza, different than antigenic drift, which refers to the natural mutation of a single strain of influenza
(E) influenza; in contrast to antigenic drift, which refers to the natural mutation of a single strain of influenza

You are concerned about the idioms involving the world "contrast" --- there's no problem there. In (A), the adverbial phrase "in contrast" is perfectly fine. The phrase "in contrast to" is also correct in and of itself, although something is funny about the way it's used in (C) & (E). Typically, the "in contrast to" prepositional phrases would be juxtaposed with the subject of the independent clause in that same part of the sentence. For example:
Obama believes in X, in contrast to Romney, who believes in Y.
That's a direct and clear use of the "in contrast to" construction.

One definite idiom --- "different from" is correct, and "different than" is 100% wrong. (B) and (D) are wrong because of that.

Big grammar idea --- when we divide a sentence with a semicolon, each side of the semicolon must be a full independent clause, capable of standing on its own as a completely sentence. Look at what follows the semicolon in (E) ---"in contrast to antigenic drift, which refers to the natural mutation of a single strain of influenza." That is not a complete sentence on its own. That is not an independent clause. That's why (E) is wrong. It can't stand on its own following the semicolon.

Now, we are down to (A) and (C). In the overall flow of the sentence, we want to compare "antigenic shift" to "antigenic drift" --- the clearest and most effective way to demonstrate this is to make them both subjects in their respective independent clauses, which is exactly what (A) does. (C), on the other hand, is very indirect, and makes it harder to see --- what's the contrast? what's the comparison? Clear, powerful, direct --- that's exactly what the GMAT SC likes, and that's what (A) gives here. (A) is by far the best answer.

Does all this make sense?

Mike