There are pros and cons to any job, and the interviewers understand this. They ask this question to see what part of your career you like the least and why. They also want to know how you overcome your least favourite part of your job and keep it from making you choose a different career path. When responding, try to keep your answer in the realm of something like long hours, little room for career advancement, or long hours on your feet, rather than difficult patients or colleagues. If you complain about your patients or colleagues, the interviewers may think you will be a challenging person to work with and may pass you over for another candidate.