Reema, a student from the northern England city of Salford in Greater Manchester, asked a question regarding his favorite politician apart from himself in BBC's children's programme 'Newsround' as part of a series on the upcoming General Election on May 7.
"If you could pick one politician apart from yourself to win who would it be and why?," 10-year-old asked Cameron.
48-year-old Prime Minister looked visibly rattled and failed to come up with an answer.
"If I thought someone else should win the election I would not be standing myself, so I can't really answer the question about who else I would like to win," he said.
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"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 other than me or I wouldn't be here, and I am quite keen on winning," he added.
"Top question - it is the best one I have been asked all election campaign," Cameron said when he was leaving.
He was also quizzed on the noisy behaviour of MPs in the British Parliament.
'Newsround' is interviewing leaders from all major parties in the lead up to next month's polls.