Swollen Black Leg

Question
It seemed to have happened almost overnight. My rat's back leg suddenly swelled up and turned black and blue. it was about as thick as my index finger, and scabbed over in places. She wouldn't use it, and it was so swollen, you couldn't even see her ankle joint, as in the foot just became an extension of her leg. She would drag it around. I called my vet for an appointment this morning, and when I just went into the cage to take her out so we could go to the vet, she had already passed on :(

Fantasia has always had skin issues with scabby rashes but that's her only medical problem. Her cagemate for over a year, whom she was very bonded to, died last month as well so I don't know if she was also just very depressed.

What happened? What is this swollen leg that took her life? Could I have prevented it? Also, she leaves behind  other cagemates... are they in danger? Thank you.

Answer
I am really sorry you lost Fantasia.  Without seeing her its pretty hard to say what it may have been but I have two educated guesses, sight unseen.  

1.  It may have been a giant hematoma, and it may have had a piece of it break off and travel to either the lungs or heart which would have killed her instantly.

2. It may have been an acute pyoderma infection that caused sepsis which is also fatal.

I am leaning more toward the first diagnosis, and either one, there would not have been anything you could have really done except get her medical treatment which you were going to do anyhow. As for how she got something like this, hard to say, but if it were a blood clot, some sort of injury or trauma may have caused it.  I do not think this would bother your other rats at all but they may mourn her loss....just keep a close on on them.