Although Brock Purdy still has to clear one more hurdle to clear the NFL's concussion protocol, the second-year quarterback was listed as a full participant for Friday's practice.

Purdy has passed four of the league's five steps in the concussion protocol. 

He reportedly experienced symptoms on the San Francisco 49ers' flight back to California late Monday night. Purdy's progress has increased optimism about his availability to play in the Niners' game against the Cincinnati Bengals Sunday.

The 23-year-old quarterback is officially listed as questionable for Sunday.

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Purdy appeared to take a hard hit to the helmet late in Monday night’s loss to the Minnesota Vikings.

Purdy participated in a portion of a walkthrough Wednesday, was limited in practice the next day and was a full participant Friday. Niners head coach Kyle Shanahan said Purdy will start if he clears the protocol. Otherwise, Sam Darnold will get the nod.

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"That's the last thing he's got to do," Shanahan said. "Just one step left. ... He was full go today; he's just got to pass that tomorrow."

To clear the fifth and final step of the league's concussion protocol, Purdy will have to be cleared for full contact by the team's doctor. A neurological consultant must also examine the quarterback. Once the team physician and an independent consultant agree Purdy no longer has concussion symptoms, he will be eligible to play in a game.

San Francisco also continues to monitor All-Pro offensive lineman Trent Williams' progress. He remains questionable to suit up this Sunday after missing last week's game with an ankle injury. Williams returned to practice on a limited basis.

"Better than last week," Shanahan said of Williams' status. "I’m still not sure. He’s got to improve here in the next 48 hours. He can go up to kickoff though."

The Niners started off the season strong, winning their first five games. The team is looking to rebound this weekend after losing back-to-back games.

The Associated Press contributed to this report.