What Has Been the Biggest Challenge (question Asked at Interview for Government Job)?

Hi I have a civil service job interview coming up. One of the standard questions they ask is What has been the greatest challenge that you've ever faced / had to overcome?
They want an answer that is relevant to the job vacancy. I've heard that the question can also be used by the interviewer to gauge what you consider to be a challenge….so tricky. Any advice would be appreciated.

Suggestion:

Tell what your challenge was but be sure and finish the story up with how you fixed the problem and put the department on top.

Example
The filing system was difficult to use
I built a cross reference system from the ground up
Now everyone in the department can easily find documents in a timely manner thus saving valuable time.

But make it to whatever your skills are and your job is. And elaborate the story a bit so you can express the difficulty of the problem.

I've had this question too.