answer (D) provides:
“Some patients who do not respond to therapies for depression may simply have received inadequate treatment, when they have, for example, been prescribed too low a drug dosage to be effective, or were taken off a drug too soon.”
There exists a problem with the general sentence structure. When you read version (D), it sounds like a run-on. Grammatically, however, the sentence is fine. The issue lies in the meaning attached to the subordinate conjunction “when” and the clause that follows it.
In general, we use the subordinate conjunction “when” and the clause that follows to talk about a subsidiary action or event that occurs at the same time with or immediately after the main sentence.
In other words, the use of “when” and the following clause implies that the events happened alongside the “receive(ing) of inadequate treatment.”
This is the wrong meaning. Instead, both being prescribed the drug at a low dose and being taken off of the drug too soon are EXAMPLES of the inadequate treatment .
If we rearrange the sentence and omit the parenthetical:
(D) “When they have….been prescribed too low a drug dosage….or were taken off a drug too soon, some patients….may have received inadequate treatment.”
The sentence implies that once they were prescribed or were taken off the drug, at that point in time, the patients received the bad treatment.
However, the author is trying to say the two events are EXAMPLES of the “inadequate treatment” that these patients may have received.
Analogous example:
Ex: “The game ended when the referee blew his whistle.”
Meaning: Immediately after the referee blew his whistle, the game was over.
The problem is that in version (D), the author is trying to use this subordinate “when” clause to provide an example of the idea described by the main sentence. This is not what this kind of “when” subordinate clause structure is designed to do.
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