Everyone has a breaking point, so this question is designed to see if your reaction and/or remedy to stress is reasonable or unreasonable. For example, you wouldn't want to say, 'One time during surgery I got so flustered and frustrated that I asked a colleague to take over because I couldn't be objective.' An answer like that will tell your interviewer that all he/she should expect from you is walking out in the middle of a surgery whenever you feel you can't take it. Instead, come off as reactive to stress but not surrendering.