Rat Tail

Question
Help! My 2 1/2 year old rattie, Boo, has developed a shriveled up tail!  The last 3 inches seem to have dried up and blacken in little more than a week.  I know he was not abused, as my mother, who babysat Boo for me, loves him as much as I.  Please, tell me what causes such a thing.  His brothers do not show any signs of similar affliction.
Your time and assistance is greatly appreciated!

Answer
His tail was injured somehow...perhaps pinched in a cage door, stepped on or bitten hard by another animal other than another rat.  This causes the tail to lose circulation in that area and the tissue becomes necrotic.  This can become infected if not treated by a vet if it involves a large portion of the tail.  if it involves just a small portion, it may simply fall off and thats that.  Can you post a photo of it here?