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: Orthopaedics
When a patient gets up from sitting position which of the following events takes place in his knee joint? 
Select and explain. 
A. Medial rotation of femur on a fixed tibia, B. Lateral rotation of femur on a fixed tibia, C. Medial rotation of tibia on a fixed femur, D. Lateral rotation of tibia on a fixed femur
A: Medial rotation of femur on a fixed tibia
Answer: A: Medial rotation of femur on a fixed tibia 
Explanation: Physiological locking occurs in extension when the femur is internally (medially) rotated on a fixed tibia.
Locking is a mechanism that allows the knee to remain in the position of full extension as in standing without much muscular efforts and is caused by quadriceps femoris.