But it did not come from a hardened inquisitor like Jeremy Paxman or John Humphrys, or from a voter at one of the workplace question and answer sessions he has attended around the country.
Instead, the question came from a Year 7 schoolgirl on the CBBC children’s news programme Newsround.
Reema, from Salford, asked the PM: “If you could pick one politician apart from yourself to win who would it be and why?”
The flummoxed premier replied: “Wow. If I could pick one politician? Would they have to be living or dead?
“Obviously, if I thought someone else should win the election I wouldn’t be standing myself, so I can’t really answer the question about who else I would like to win.
“There are lots of candidates around the country I am very enthusiastic about.
“I am afraid it is too difficult to say I would like someone else to win rather than me otherwise I wouldn’t be here. I’m afraid I am quite keen on winning.”
As Mr Cameron got up to leave he said: “Top question - I think it’s the best one I have been asked all election.”
Presenter Ayshah Tull said that Newsround viewers had written in to say that they would not be allowed to behave in school like MPs do in the House of Commons.
Mr Cameron agreed the noise at Prime Minister’s Questions was “terrible” and MPs’ behaviour was not always “great”. But he said when his children Nancy and Elwen came to watch the final PMQs of the Parliament from the visitors’ gallery last month, “they weren’t impressed by the noise, but they could sort of see the point that you’ve got to be out there answering the questions and facing the music”.
He added: “You’re not going to change it so fundamentally that people sit in absolute silence. It’s always going to be a bit high-octane. We can improve it, but I wouldn’t scrap it.”
Mr Cameron was also asked if it was “scary” having to appear on TV all the time as Prime Minister. He said that television appearances were not an issue as “you get used to that”, but acknowledged there were aspects of the job which were “quite worrying and difficult”, such as sending troops overseas and dealing with hostage situations.