''NOT A REAL HOSPITAL,'' reads the sign in front of the four-story, dirty white building where NBC's Thursday-night sitcom Scrubs is filmed. Tell that to the man with the head wound. ''When we started shooting here, a guy with a bloody head came racing in because he thought the place was still a real hospital,'' recalls creator/exec producer Bill Lawrence. Up until five years ago, the Studio City building was the functional North Hollywood Medical Center, and to transform it into the fictional Sacred Heart, ''we added some rust spots on the walls to show a hospital in decline,'' says former Scrubs production designer Cabot McMullen. His crew also scuffed up the flooring and collected some of the worst art in America for the walls. Star Zach Braff (J.D.) points out that even the gift shop is fully stocked: ''You can pick up a religious snow globe. For anyone who wants to send a postcard from the hospital, they've got pictures from the Eiffel Tower. You can also pick up film to take pictures of your sick loved ones.'' Gift shops aside, Donald Faison (Turk) says the set can be a tad disconcerting: ''You don't ever want to go in the basement because that's where the morgue is. And I stay away from the psych ward. That's where they keep the writers.''