This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Misc., Subject name: Physiology
Which one of the following is absent in smooth muscles? 
Select and explain. 
A. Actin, B. Myosin, C. Troponin, D. Myosin adenine triphosphatase
C: Troponin
Answer: C: Troponin 
Explanation: (C) Troponin> Smooth muscle have no troponin or tropomyosin, but they have actin and myosin.> They have no sarcoplasmic reticulum, No T-tubule.> No Z lines. The contraction is stimulated by extracellular ca+.> Latching - is the characteristic of smooth muscle. Even after the dephosphorylation of myosin, the cross bridges continue to attach in the same position for some time. There is no fixed tension relationship.