This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General, Subject name: Physiology
The major source of calcium for contraction of skeletal muscle is: 
Select and explain. 
A. ECF, B. Cytosol, C. Mitochondria, D. Sarcoplasmic reticulum (SR)
D: Sarcoplasmic reticulum (SR)
Answer: D: Sarcoplasmic reticulum (SR) 
Explanation: Ans. D. sarcoplasmic reticulum (SR)Skeletal muscle depends on the SR as the major source of calcium. Cardiac muscle and smooth muscle depend on influx of calcium from ECF and Ca-induced calcium release for contraction.