Sore on rabbits back foot

Question
I noticed the other day that my rabbit has a large sore on his back foot.  He is a domestic rabbit who got out of his cage and was loose for about 3 days.  I was researching this sore on the internet and some of the symptoms seem to be like that of those of ringworm.  It is fall here and he is loosing alot of his fur, but I assumed that he is shedding his summer coat and getting a fall one.  The sore is about the size of small gumball and red and purple in color.  Can you give me any information on this?

Answer
That sounds more like a sore hock.  It's just on the bottom of the foot, right?

There's information about this at http://homepage.mac.com/mattocks/morfz/rabrefs.html#hocks

He could also have stepped on something that caused it to become sore, or it could be ringworm.

And yes, every rabbit I know seems to be dropping their coat lately, so it is shedding season.

Kim