The phrase "a single topic that" is correct and usable in written English.
You can use it when referring to one specific subject or theme that is being discussed or analyzed.
Example: "In this essay, I will focus on a single topic that has significant implications for our understanding of climate change."
Alternatives: "one specific subject that" or "a particular theme that".
There's something gloriously matter-of-fact about her, and during a conversation of more than 90 minutes there wasn't a single topic that made her flinch.
The notable exception to The Economist's no-byline rule, at least in the weekly issue, is special reports, the collections of articles on a single topic that appear in the newspaper every month or so.
Social news site Reddit announced on Wednesday that the company would be banning subreddits – areas of the site dedicated to a single topic – "that allow their communities to use the subreddit as a platform to harass individuals when moderators don't take action".
In my 28 years as a journalist, I cannot think of a single topic that would have been more justified being debated publicly in a democratic society than Edward Snowden's, Glenn Greenwald's and the Guardian's revelations of these last few months.
Feedburner is testing a new product called "Networks" which are groups of blogs on a single topic that are using Feedburner to manage their RSS feed.
According to Guldenschuh's Heartland Institute colleague, Rob Natelson, early conventions of states discussed a single topic that state legislatures defined ahead of time.
"it was a treasure trove," one that touched on "every single topic that Darwin was thinking about with respect to what we today call genetics and heredity.
But last week, the hour disappeared in a single topic, one that has consumed American Jews lately: What to do? Buy Israeli products, one young professional suggested, pointing the others toward the imported hummus at the Jewel supermarket on the city's North Side.
We recommend choosing a single topic so that students can later compare their recollections and interpretations.
We choose to study questions about a single topic so that the human judges could more easily refresh their biology knowledge before judging the questions.
This doesn't mean that you keep writing on a single topic, just make sure that the topic somehow relates to a subject in which you have shown some expertise to your readers already.
