This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
Calcium initiates a skeletal muscle contraction by 
Select and explain. 
A. Binding to tropomyosin, B. Being released primarily from the sarcolemma during an action potential, C. Binding to troponin, D. Covering action binding sites
C: Binding to troponin
Answer: C: Binding to troponin 
Explanation: During excitation-contraction coupling in a skeletal muscle fiber, calcium is released from the sarcoplasmic reticulum (not the sarcolemma) and binds to troponin (not tropomyosin or myosin). The binding of calcium to the troponin causes the troponin to shift the position of the tropomyosin to which it is attached, thus uncovering the actin binding sites for binding with myosin.