A 19-year-old man was killed and five people were wounded in four separate drive-by shootings that may have been gang-related, authorities said. Sergio Rene Ramirez was shot from a passing car as he walked out of a house in the 300 block of North Ditman Avenue, police said. He died later at County-USC Medical Center. In an unrelated incident, authorities said that Steve Castro, 16, was seriously injured in an attack in the Mount Washington area. Dwayne Murrilo, 21, was in guarded condition after being shot in the face while standing on a sidewalk in the 1500 block of West 80th Street. And three unidentified teen-agers were treated for minor gunshot wounds in a fourth drive-by shooting that occurred as they stood next to a car outside 2500 West 42nd St., police said.