(Portland, OR)  --  Portland police say they've arrested a man who struck a pedestrian in a crosswalk, then drove away to drop off his girlfriend, who was riding with him.  Authorities say the crash happened Monday at the intersection of Northwest 15th Avenue and West Burnside Street, leaving the pedestrian with a broken neck and a bruised lung.  Police say Nam N. Vu told investigators he heard a thud, and planned to return to the scene after dropping off his girlfriend.  Vu is facing charges that include second-degree assault and reckless driving.