This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Joints, Subject name: Anatomy
Which is an unlocker of knee? 
Select and explain. 
A. Popliteus, B. biceps femoris, C. Sartorius, D. Gastrocnemius
A: Popliteus
Answer: A: Popliteus 
Explanation: ANSWER: (A) PopliteusREF: Gray's anatomy 39th ed p. 1465,1482-83* Locking or screw home mechanism: Conjuct medial rotation of femur on tibia in later stages of extension due to quadriceps femoris locks the knee joint.Unlocking of knee joint: At beginning of flexion from full extension with foot flexed, lateral rotation by Popliteus muscle unlocks the joint.