What could be the cause of a cat licking paw to death.

[KYS]

rg_girlca,
Don't worry about hurting my feelings.
I agree with you.
Originally Shadow was my nieces cat,
but when my niece moved out on her own,
she left Shadow behind.

I heard she was going to put some human
cream for people on her paw. I think it was
a hydrocortisone cream.
The vet had told her on the last visit,
that if the problem persists,
the cat would need to come in for testing etc.
(which my sister doesn't want to do)

Edited: I Just got off the phone with my sister,
she agreed to take Shadow back to the vets.