HOUSTON — To many, it was the most loaded question at Thursday night’s Democratic presidential debate.

“Are you forgetting what you said two minutes ago?” Julián Castro asked Joe Biden in a heated exchange over health care. The crowd inside the debate hall reacted with a collective gasp, as if they had witnessed a scandalous twist on an afternoon talk show.

Then, Castro repeated himself. Twice.

And just like that, an uncomfortable topic that has lingered just below the surface of the primary for months erupted into clear view before millions of Americans on national television: Are there issues with Biden’s mental acuity and memory? Or not?

Castro’s assertion that Biden, 76, contradicted what he had said earlier in the debate about his health-care plan didn’t square with the former vice president’s words, raising the possibility that Castro’s aggressiveness could backfire.

The former vice president had just finished defending his plan to expand the Affordable Care Act with an optional public insurance program.

Biden said he was for an optional public insurance program he dubbed “Medicare for Choice.”

“Anyone who can’t afford it gets automatically enrolled in the Medicare-type option we have,” he said. Biden later said that people who lose their jobs and associated insurance automatically can buy into the plan. “You don’t have — no preexisting condition can stop you from buying in. You get covered, period.”

Castro attacked. (truncated)