Here's the
official explanation provided by the GMAC for this question:
The point of this sentence is that in general, when or shortly after a child whose parents have allergies develops a minor infection, the child is likely to develop an allergy. The underlined portion of the sentence needs to clearly express the order of the events described and their implicit causal relationship.
Option A: This phrasing with the static verb
to be fails to convey the intended idea that the children are likely to become allergic after developing a minor infection. Because
having once is set off by a comma, the sentence confusingly seems to say that every child of parents who have allergies has once developed a minor infection.
Option B: The future tense
will develop is inappropriate for the sentence's generalized description of past, present, and future cases. The
will in the main body of the sentence indicates that the state of being allergic occurs in the future relative to the event of developing a minor infection. But the infection's development is not being said to occur in the future relative to any other event described, so the future form
will develop should not be used.
Option C: The phrasing with the static verb
to be and the conjunction
as suggests that the children are allergic while developing a minor infection, but not necessarily after the infection ends. This phrasing fails to express the sequence of events: first the children develop an infection, and then as a result they become allergic.
Option D: The lack of a comma makes this phrasing hard to parse–
themselves may be read incorrectly as modified by
having once developed. The phrasing fails to convey the intended idea that having developed an infection causes the children to become allergic; the two events are not clearly connected.
Option E: Correct. The dynamic verb
to become clearly conveys the intended idea of developing an allergy rather than merely having an allergy. The conjunction
once and the simple present
develop appropriately express the intended general time sequence: in many past, present, and future cases, an allergy arises during or shortly after a minor infection.
The correct answer is E.
Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.