Don't know about the example but only option A is parallel.
.....having, for example been prescribed a drug as a dosage too low to be effective or having been.......

sulabh wrote:Don't know about the example but only option A is parallel.
.....having, for example been prescribed a drug as a dosage too low to be effective or having been.......

Do we not say that having been is wrong in GMAT .What are the rules for defining the correct usage of having been/

Final solution at one place:

Important: The purpose of this post (and all the other posts by me) is to give a complete solution to all GMAT-Prep Verbal questions at one place. Sometimes students have to wade through dozens of posts to get to the final answer. My posts will give one complete and crisp solution required to arrive at the correct answer by eliminating the wrong ones. Some of the content in these posts may have been taken from various other sources (discussion forums).

Some patients who do not respond to therapies for depression may simply have received inadequate treatment, having, for example, been prescribed a drug at a dosage too low to be effective or having been taken off a drug too soon.
(A) having, for example, been prescribed a drug at a dosage too low to be effective or having been
(B) having, for example, a drug prescription that was ineffective because the dosage was too low, or being
(C) as, for example, having too low of a dosage of a prescribed drug for it to be effective, or being
(D) when they have, for example, been prescribed too low a drug dosage for it to be effective, or were
(E) for example, when they have a drug prescription with a dosage too low to be effective, or been

The best way to attack this problem is to remove 'for example' from each of the choices.

A. having been prescribed a drug at a dosage too low to be effective or having been
B. having a drug prescription that was ineffective because the dosage was too low, or being
C. as having too low of a dosage of a prescribed drug for it to be effective, or being
D. when they have been prescribed too low a drug dosage for it to be effective, or were
E. when they have a drug prescription with a dosage too low to be effective, or been

A is parallel 'having been || having been' ... the connector word is 'or' ... as we are joining two similar ideas (and only two ideas), we should not use a comma. This alone is sufficient to eliminate B, C, D, and E.

In B, 'having a drug prescription (refers to patients) can't be parallel to being taken off (refers to a doctor, possibly) ... as the subjects are different, the two are not parallel.

In C, 'having too low of a dosage (refers to patients) can't be parallel to being taken off (refers to a doctor, possibly) ... as the subjects are different, the two words are not parallel.

D is not parallel ('have been' is not parallel to 'were')

E is not parallel ('have a prescription' is not parallel to 'been taken off')

Correct: A