Oregon State Police are investigating a fatal, single-vehicle crash on Interstate 84 in Gilliam County.  The crash happened shortly after midnight Sunday near milepost 140, east of Arlington.  State police say a white Toyota Avalon driven by 24-year-old Adhishwar Ramanathan of Umatilla was traveling eastbound when, for an unknown reason, the vehicle left its lane and struck the end of a guardrail.  The Toyota came to rest facing westbound in the eastbound lanes.  Ramanathan was pronounced dead at the scene.  The highway remained open during the investigation.  The Gilliam County Sheriff’s Office, North Gilliam County Rural Fire Protection District, and Oregon Department of Transportation assisted Oregon State Police at the scene.