Tendinous xanthomas are a deposition of lipids into the macrophages of the skin overlying tendons or ligaments. Most commonly, you will see these on the hands, feet and overlying the Achilles tendon, but they can also show up in other locations such as the elbows, knees, etc. They are associated with sky-high lipids, and are commonly considered to be a cutaneous manifestation of internal disease. How to tell these aren't gouty tophi? They are too specifically associated with the tendons and joints, especially on the dorsal hands. How about calcinosis cutis? Too widely distributed, once again too specifically over the tendons to be calcinosis cutis. So, everyone understood these were depositions of a substance not native to the skin, but only one of you (Gwen) nailed it.