Final answer:
The correct answer is option c. Discomfort under the kneecap while going down steps is most likely associated with the Patellofemoral joint, the articulation between the patella and the distal femur.
Step-by-step explanation:
When experiencing discomfort under the kneecap while going downstairs, the most likely joint involved is C. the Patellofemoral joint. This joint includes the articulation between the patella (kneecap) and the distal femur (thigh bone). The patella serves to protect the tendon of the quadriceps femoris muscle and assists with the knee's flexion and extension.
When the knee joint is in action, especially during activities like descending steps, the patella glides along a groove on the distal femur. If there is discomfort, it is often due to issues like chondromalacia patella or patellofemoral pain syndrome, which are conditions specifically affecting the patellofemoral joint.