DEL MAR, Calif.—Kentucky Derby winner Medina Spirit held off Rock Your World in the stretch to win the $100,000 Shared Belief by 1 1/4 lengths on Aug. 29 at Del Mar for embattled trainer Bob Baffert. Medina Spirit raced for the first time since finishing third as the 9-5 favorite in the Preakness on May 15, days after the colt’s medication positive from the Derby on May 1 was revealed by Baffert. Ridden by John Velazquez, Medina Spirit ran a mile in 1:37.29. He paid $3.80, $2.40 and $2.10 at 4-5 odds. “It’s a relief. I was a little nervous today,” Baffert said. “It just shows the class that he has and determination. This is the reason he won the Kentucky Derby, he’s that good a horse.” Sent off at 8-5, Rock Your World returned $2.80 and $2.20. The colt, who beat Medina Spirit in the Santa Anita Derby in …