What Causes An Open Sore That Scabs And Falls Off On A Thigh?

My wife has a open sore on her left thigh about three inches down from her vagina. She has seen a dermatologist but because of the vacation week after Christmas she has not heard anything. The sore looks like a blister that is open to start than scabs over than the scab falls off. This has been occurring for about six weeks. Could you give me some possible conclusions. Thanks. Brent