Benefield is charged with causing injuries to little Lucus that led to his death in February 2012.
Doctor Ronald Wright flew in from Florida to testify for the defense.
He says after reviewing Lucas' medical records, he concluded that the infant did not die form shaken baby syndrome, which a witness to the prosecution testified to last week.
He showed autopsy photos of Lucas' neck and skull, which he says shows no red areas to indicate bleeding.
Doctor Wright did say the four month olds arm and rib fractures were inflicted and that he showed classic symptoms of child abuse.
Copyright 2015 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.