I am updating my article on the usefulness of the SDMA kidney test.
If your vet  has followed SDMA levels in your pet, perhaps you would 
share those values with me. RSH email
  I drew this fanciful image to show you where acral lesions commonly occur. The green circle surrounds two other locations where this problem is common. The amount of scaring and contraction surrounding the wounds tell me that the problem has been present for quite some time. Because this dog is young, large, lean, in apparent good-health and quite apprehensive, I would be suspicious that this dog's problem had a psychological basis rather than an organic one. But every dog that suffers from acral lesions like these should have a complete health work-up in any case, to rule out organic (physical) disease.