^Unless it’s a long chain (at the workplace in a 30+ email chain, the rules are a bit different), I’d assume the emails went like this:
Subject: Psychology 101A
Dear Professor:
I had a question regarding the essay due on Tuesday. I looked in the syllabus, but did not see any information about how long the essay was expect to be. Could you expand upon that?
Thank you,
OP
Subject: Re: Psychology 101A
Dear OP:
I have an answer.
Best,
Professor
Subject: Re: Psychology 101A
Could you please expand?
When the appropriate response would be something like this:
Subject: Re: Psychology 101A
Dr. X,
Could you please expand?
OP