JUST LISTED: Street-Level Medical Office w/ Separate Outside Entrance at 3755 Henry Hudson Parkway    This approximately 600-square-foot professional suite has been used as a pediatric endocrinologist's office. Located at The Imperial (3755 Henry Hudson Parkway), a post-war co-op building in the affluent Riverdale section of the Bronx, it has 24-hr. doormen. The space is already finished with custom built-ins (including desks, sinks and cabinetry) and has its own outside street entrance.     Features include: a waiting room with reception area; 2 windowed examination rooms; walk-through storage and filing room; updated bathroom; east and west exposures; corner unit. Monthly Maintenance $742.    Attractive and highly visible building across the street from MTA bus stops at West 239th Street and Henry Hudson Parkway. Conveniently located for Riverdale and Johnson Avenue shops and eateries.