What could cause a swollen knee with bruising down to the ankle?

Swelling and bruise: Trauma or broken blood vessel, or clotting deficency.

Swelling to ankle: Can be definitive for a clotting disorder such as taking Aspirin or anticoagulant drugs such as Plavix or coumadin (warfarin). Trauma or injury could also play a role. Seek medical attention especially if you have other medical problems or take medications such as steroids for something else that could make this problem worse.