In his research paper, Dr. Frosh, medical director of the Payne Whitne

Let's take a look at the sentence using the correct answer choice:


Notice that "medical director of the Payne Whitney Clinic" is just a comma-separated modifier that gives us more information about Dr. Frosh. The comma after "clinic" is simply the second of two commas that separate the modifier from the noun it modifies ("Dr. Frosh"). If we remove that modifier, we could also remove the two commas:

"In his research paper, Dr. Frosh distinguishes between mood swings, which may be violent without being grounded in mental disease, and genuine manic-depressive psychosis."

Now it's easier to see that "Dr. Frosh" is the subject and "distinguishes" is the verb.

Notice that "which may be violent without being grounded in mental disease" modifies "mood swings," and the opening modifier ("In his research paper") modifies the entire main clause (telling us where Dr. Frosh distinguishes between...). Removing those modifiers leaves us with the following:

"Dr. Frosh distinguishes between mood swings and genuine manic-depressive psychosis."

This is the core of the sentence: "Dr. Frosh distinguishes between [X] and [Y]." Everything else is just additional modifying information that, while essential to the meaning of the sentence, can be removed without impacting the core structure of the sentence.

I hope that helps!