Watch: Top 5 highlights from the Republican debate

>> PHOTOS: 4th GOP debate

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1. Carson fires back about questions surrounding his life story. When Neil Cavuto asked neurosurgeon Ben Carson to weigh in on the recent media scrutiny of his past, Carson opened with a joke.

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2. Paul, Rubio spar over national defense. In what Facebook dubbed the "top social moment of the debate," Kentucky Sen. Rand Paul and Florida Sen. Marco Rubio clashed over their tax plans – specifically on the issue of military spending.

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3. Cruz doubles down on cutting the Department of Commerce. When Texas Sen. Ted Cruz discussed his tax plan, he tried to name five agencies he would cut. But something didn't add up.

Turns out, that's just four agencies; he listed the Department of Commerce twice. According to Cruz's website, the elusive fifth agency is the Department of Education.

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4. Kasich, Bush slam Trump's immigration plan. Real estate mogul Donald Trump again outlined his views on illegal immigration – but some of the other candidates weren't impressed.

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5. Fiorina jabs at Trump after Vladimir Putin comment. After the businessman said he and the Russian president were "stablemates" for a "60 Minutes" interview, Trump added that he would support Putin's intervention in Syria.

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