Patella (Knee cap)
The structure indicated is the patella bone.
The patella (knee cap) is a triangular shaped bone that is encapsulated within the quadriceps femoris tendon and is the largest sesamoid bone in the human body. It is located anteriorly to the knee joint and attaches via its inferiorly directed apex to the proximal tibia by the patellar ligament.
The base of the patella provides a broad surface for the attachment of the quadriceps femoris muscles. The patella functions to transmit the forces from the powerful thigh muscles to the leg and is particular important in knee extension.
Learn more about the anatomy of the knee joint in this tutorial