My answer applies to any challenge/weakness question.
Any question that you answer always need to circle around to the answer of why you would be a good physician and possess the qualities of one.
If you answer the question with "Problem is X", that fails to address the intent and opportunity of the question. The only thing you demonstrated is that you are self aware of problems. Being aware of a problem means nothing for your character without action.
The important part of the answer needs to be the "So what are going to do about Problem X?" Here's an example of a proper answer:
"The biggest challenge is definitely my X. However, I have been doing Y and Z to improve the X, and I've been seeing great improvements. Example, A and B."
An answer like that demonstrates willingness to improvement, strong self analysis skills, plan making, momentum, and positive character.
Any time you see a question:
"What is your challenge/weakness/etc in anything?", you need to always mentally change it to:
"What is your challenge/weakness/etc in anything (and how did you address it)"
