This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
Which of the following muscle acts as unlocker of knee? 
Select and explain. 
A. Gracilis, B. Popliteus, C. Saorius, D. Biceps femoris
B: Popliteus
Answer: B: Popliteus 
Explanation: Popliteus rotates the tibia medially on the femur or, when the tibia is fixed, rotates the femur laterally on the tibia. At the beginning of flexion of the fully extended knee, lateral femoral rotation by popliteus muscle unlocks the joint.Must know:Locking of knee joint is due to the the action of quadriceps femoris, that brings about medial rotation of femur on tibia in later stages of extension. Ref: Snells, Clinical anatomy, 7th Edition, Page 650.