How can I run describe-key on the C-u C-x = key sequence?

I know that `C-u C-x =` shows the font details of the face at point. How can I do a describe-key for this key? If I try to do `C-h k C-u C-x =`, Emacs responds as soon as I have typed `C-h k C-u` and shows that `C-u` is bound to `(universal-argument)`. The `C-x =` keys after `C-u` gets ignored. How can I see which function is invoked by `C-u C-x =`?

The `universal-argument` binding sets the prefix argument to whichever sequence follows. Therefore what you are looking for is simply: `C-h k C-x =` Which tells us: [...] In addition, with prefix argument, show details about that character in *Help* buffer. See also the command `describe-char'. This, in general, is what you are looking for -- details of how the command varies when given a prefix argument.

Thanks!