EVANSTON, Ill.(AP) Penn State's defense clamped down after Northwestern lost starting quarterback Mike Kafka .

In the fourth quarter, the Nittany Lions ' offense kicked in.

Evan Royster bolted 69 yards for a TD to cap a three-score spurt in the final period and No. 12 Penn State pulled away to a 34-13 victory Saturday night.

''You got to understand that a kid like that quarterback (Kafka) is out of there, it's a big loss,'' Penn State coach Joe Paterno said. ''Kafka's a heck of a football player. We played them more aggressively in the second half and did a better job with the pass rush, regardless of whether Kafka was in there or some other kid.''

Daryll Clark threw a 53-yard TD pass to Derek Moye before Royster broke loose for the third of three Penn State touchdowns during a span 3 minutes, 40 seconds of the fourth quarter that turned a tight game into a rout.

''I was just praying Daryll was going to hand the ball off, because it was one of those ones where he could throw it out or hand it off,'' said Royster, who ran for 118 yards on 15 carries. ''I knew it was going to open up. After that I just had to run as fast as I could.''

Penn State (8-1 4-1 in the Big Ten) trailed 10-3 in the second quarter when Kafka was forced out with a left leg injury. And the Nittany Lions were still behind 13-10 at the half. But they shut out the Wildcats and backup QB Dan Persa in the final two quarters and gave Paterno his 391st win.

''I think that affected them. I think the kid came in and did a good job. Kafka was really playing well in the first half. He had us on our heels,'' Penn State linebacker Sean Lee said.

Behind the strong running of Royster, Penn State moved in for the go-ahead touchdown early in the final quarter on a 2-yard run by Brandon Beachum that made it 20-13.

When the Nittany Lions got the ball back, Clark rolled right and found Moye behind Northwestern's secondary for the 53-yard TD pass, making it 27-13 with 10:16 to go.

After Penn State forced another punt, Royster broke through the Northwestern defense on first down and sprinted to the end zone.

Northwestern (5-4, 2-3) finished with 371 yards total offense after having 246 in the first half.

Kafka injured his leg midway through the second quarter. He'd been sacked and then completed a pass before he was replaced by Persa. When he left, he'd completed 14 of 18 passes for 128 yards and carried eight times for 42 more.

''When Mike was in there I don't think they stopped us. I think we stopped ourselves a couple of times,'' Northwestern coach Pat Fitzgerald said.

Persa, whose mother and sister attended Penn State, completed 14 of 23 passes and rushed for 42 yards on 14 carries, but couldn't get the Wildcats to the end zone. He was sacked four times.

''No excuses. I made some mistakes. ... I didn't get it done,'' Persa said.

There was no word on what type of injury Kafka had or how long he would be out. He was on the sideline during the second half and even pedaled an exercise bike, but did not return. He entered the game leading the Big Ten in passing yards and total offense per game.

''If he was healthy, I'd put him back in there,'' said Fitzgerald.

Clark was 22 of 31 for 274 yards.

''We noticed there were a couple of voids in their defense. ...We knew there would be a big play for us,'' Clark said.

Clark led the Nittany Lions on a 93-yard drive in the second quarter, going 5 for 5 for 64 yards and also carrying for 12 yards. After a pass interference call on Northwestern, Clark hit Curtis Drake for 13 yards and then scored from the 2 - his 20th career rushing TD, tying Michael Robinson 's school record - to make it 10-10.

Persa then got the Wildcats in position for a go-ahead score but Jared Odrick blocked Stefan Demos ' 37-yard field goal attempt. Demos got another shot and hit a 45-yarder with three seconds left in the half to give Northwestern the 13-10 lead.

Collin Wagner kicked a 23-yard field goal to tie it in the third quarter.

After the team's traded first-quarter field goals, Kafka and the Wildcats grabbed the lead in the second quarter.

Kafka completed four straight passes in a hurry-up offense, had a 13-yard keeper on a key third down, and carried the final 7 yards to complete an 80-yard TD drive that put Northwestern up 10-3. It was just the second first-half TD allowed by Penn State's rugged defense all season.

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