Antigenic shift refers to the combination of two different strains of influenza, different than antigenic drift, which refers to the natural mutation of a single strain of influenza


The underlined portion above is wrong. OE given by MGMAT is as below.

"This choice uses the incorrect comparison phrase "different than"; the correct phrase is "different from." In addition, the simple comma between "influenza" and "different" provides an inadequate transition between the two parts of the sentence; the addition of a conjunction such as "and is" (e.g., "... influenza, [and is] different ...") is necessary here."


I understand the explanation. But if correct 1st problem by using "different from", i think sentence will correct as part of the sentence starting with "different from" will become adjective phrase.




Please correct.

Antigenic shift refers to the combination of two different strains ofinfluenza , different from antigenic drift, which refers to the natural mutation of a single strain of influenza

nope - if you make just that one fix, then the sentence still has problems.

specifically:
that sort of modifier - "different from antigenic drift, ..." - (which is called an appositive modifier, if you care about linguistic terminology) - is generally attributed to the immediately preceding noun.
so, depending on whether you take the "immediately preceding noun" to be the literally preceding noun or the entire noun phrase, you're saying one of the following things:
* (2 different strains of) influenza is what's different from antigenic drift
* the combination of 2 different strains of influenza is what's different from antigenic drift
the first is just wrong, and the second is wrong enough to be incorrect from the standpoint of sentence correction. remember, you should be interpreting these things very literally: the TERM "antigenic shift" differs from the TERM "antigenic drift". (the reason you know the sentence is about TERMS is because of its use of "refers to", which we wouldn't use if we were saying antigenic shift IS the phenomenon.)

to maintain parallelism, you need a sentence that says unambiguously that antigenic shift is what's different from antigenic drift. to do that, you need a conjunction (as mentioned above), because antigenic shift is the subject of the first sentence.