**Core Concept**
The popliteus muscle is a small intrinsic muscle of the knee that plays a key role in unlocking the knee joint during flexion, particularly in the early phase of movement when the knee is fully flexed and the tibiofemoral joint is locked.

**Why the Correct Answer is Right**
The popliteus muscle originates from the lateral aspect of the femur and inserts on the posterior surface of the lateral condyle of the tibia. It is the only muscle that can rotate the femur posteriorly and unlock the knee joint by rotating the femur slightly posteriorly, allowing the tibia to move past the femur. This action is essential during the initial phase of knee extension after full flexion, especially when the knee is locked in a flexed position. It acts as a **primary unlocker** of the knee joint, preventing mechanical jamming.

**Why Each Wrong Option is Incorrect**
Option A: Gracilis – This is a long, slender muscle of the medial thigh that assists in hip flexion and knee flexion, but it does not contribute to unlocking the knee joint.
Option C: Saorius – This is a misspelling of *Sartorius*, a long muscle of the anterior thigh involved in hip flexion and lateral rotation, not knee unlocking.
Option D: Biceps femoris – This is a hamstring muscle that assists in knee flexion and hip extension, but it does not unlock the knee joint.

**Clinical Pearl / High-Yield Fact**
The popliteus is the only muscle that unlocks the knee during extension; its action is often overlooked but critical in joint mechanics, especially in patients with stiff or locked knees. It is also a key muscle in the **knee joint's mechanical unlocking** and is involved in early gait mechanics.

✓ Correct Answer: B. Popliteus