**Core Concept**
The locking mechanism of the knee joint is a complex process involving the interplay of various muscles, ligaments, and bony structures. It is essential to understand the anatomy and physiology of the knee joint to appreciate its locking mechanism. The quadriceps and hamstring muscles play a crucial role in this process.

**Why the Correct Answer is Right**
When the knee is fully extended, the quadriceps muscles contract, causing the patella to move upward and lock into the intercondylar notch of the femur. This locks the knee joint in place, preventing excessive flexion or extension. The locking mechanism is also influenced by the position of the tibia and the tension in the ligaments surrounding the knee joint.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the muscle or structure responsible for knee locking.
**Option B:** This option is incorrect because it refers to a different joint, the elbow, which has a distinct locking mechanism involving the olecranon process and the medial and lateral epicondyles.
**Option C:** This option is incorrect because it mentions the popliteus muscle, which actually unlocks the knee joint by rotating the femur on the tibia.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the quadriceps muscles are responsible for knee extension and locking, while the hamstring muscles are responsible for knee flexion and unlocking.

**Correct Answer:** D.