A natural way to say it is: “bruising extending from the knuckles to just below the wrist”.

Other clean options are:

  • “Bruising across the back of the hand from the knuckles down to just below the wrist.”
  • “Visible bruising from the knuckles to the lower wrist area.”
  • “Bruising over the hand and wrist region.”

If you want it to sound more medical, you could say “contusion involving the knuckles and extending to the distal wrist”. If you want it to sound more plain or casual, “bruising from the knuckles down toward the wrist” works well.