It was the catalyst that set Donald Trump on a collision course with defeat.
And it was a question so big and bad that the Republic candidate was unable to recover from it.
When debate host Lester Holt turned attention to Trump's refusal to release his tax returns, Clinton pounced on what happened next.
"I will release my tax returns against my lawyers' wishes, when she releases her 33,000 emails that have been deleted. As soon as she releases them, I will release my tax returns and that is against ... my lawyers say don't do it," Trump said.
Clinton then hit him where it hurt.
"For 40 years, everyone running for President has released their tax returns. You can go and see nearly I think 39 or 40 years of our tax returns," she said.
"Everyone has done it. We know the IRS has made clear there is no prohibition on releasing it when you're under audit. You have to ask yourself why won't he release his tax returns?
"If he has paid zero (tax), that means zero for troops, vets, schools or health. I think probably he is not all that enthusiastic about having the rest of our country see what the real reasons are because it must be something really important even terrible that he is trying to hide."
Dr John Hart, a research fellow at the ANU School of History and a specialist in American politics and elections, said this was a key point which turned the debate on its head.
"Trump started off quite well with the trade issue and his answer may not have been correct but it was effective," he said.
"But when he couldn't answer the tax question Hillary Clinton really went to town on him.
"That question affected his ability to string a sentence together after that."
Dr Hart said Clinton's response and tactic was so effective Trump wasn't able to recover and turned the rest of his performance "into a farce."
Trump was also left stumped when asked why he continued to deny President Obama was born in the US up until a few weeks ago when it was long ago proven he was born in the US.
Trump was the leader of the "birther" movement, whose members believe Barack Obama wasn't born in the United States.
Obama produced his birth certificate in 2011 proving he was a US citizen.
"Mr Trump, for five years you perpetuated a false claim of the nation's first black president wasn't a natural born citizen. You questioned his legitimacy," Holt said.
Trump replied by claiming a Clinton ally, Sid Blumenthal, had raised the birther conspiracy first and he did the world a favour by getting the proof.
However, he didn't answer Holt's question on why he changed his mind.
"Nobody was pressing it. Nobody was caring much about it. I figured you'd ask the question tonight, of course," Trump said. "But nobody was caring much about it. But I was the one that got him to produce the birth certificate and I think I did a good job".
Trump also stumbled over the Iraq issue and denied he had ever supported the war.
"You had supported the war in Iraq before the invasion. What makes your judgment ..." Holt began to ask before Trump said: "I did not support the war in Iraq."
"That is a mainstream media nonsense put out by her because she frankly, I think, the best person in her campaign is mainstream media."
Holt's argument was based on an interview he gave to Howard Stern in 2002 when he was asked if he was in favour of the war. He replied: "Yeah, I guess so."
Dr Hart said the fact Trump tied to turn the birther issue on its head was simply ludicrous and made him look farcical.
"Trump did try and make the debate personal and when he did it fell flat or backfired," he said.
Chief Executive Officer at the United States Studies Centre at the University of Sydney Professor Simon Jackman said Trump came off worse for wear and his answers were far from convincing.
Prof Jackman said Trump also showed his true temperament by cutting Clinton off and talking over her.
"He had some cutaway moments and had some valid criticisms," he said.
"But the dominant image I have is one of him being quite surly with a sour look on his face as he was talking over a woman."
Prof Jackman said Trump's claim that he did the world a favour by asking Obama to prove where he was born did him no favours.
"For him to come out and say Clinton started it and he finished it just blew me away," he said.
Prof Jackman said he believed the tax question was a strategic move by the Clinton camp to throw Trump off course.
"It was a deliberate part of her strategy to make him go places where questions are put back onto Trump and was part of a plan to make him angry, more flamboyant and more surly," he said.
He said when Trump tried to deflect the question by drawing attention to the issue of her emails, she simply threw the question back at him by asking what he was trying to hide.