Based on a game by Mink's M no Violet division. 
 
Before Ren Nanase was ever a night shift nurse, she was a sexy coed at a nursing school. Hearing rumors that one of the faculty is a rapist, she does exactly what any model citizen would do and confronts him about it. Unfortunately, things go downhill from there, and she winds up being his sex slave, getting a crash course in depravity. But in the middle of that perversion, could there be a hint of purity? 
Note: There are 2 Nanase Ren OVAs produced totally different studios, but both feature the same character. The other one is the "Kranke Nanase Ren" (夜勤病棟　Kranke　七瀬恋) bit, which is a special for Yakin Byouto

Night Shift Nurse: Ren Nanase