This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Muscle Physiology, Subject name: Physiology
[{'content': 'True regarding excitation contraction coupling in smooth muscle is \nSelect and explain. \nA. Presence of troponin is essential, B. Sustained contraction occurs with high calcium concentration, C. Phosphorylation of actin is required for contraction, D. Presence of cellular calcium is essential to cause muscle contraction', 'role': 'user'}, {'content': 'Answer: D \nExplanation: Ans. (d) Presence of cellular calcium is essential to cause muscle contraction(Ref: Ganong, 25th ed/p.116)Troponin is absent in smooth muscleSustained contraction (latch) occurs with less use of ATPPhosphorylation of myosin is required for smooth muscle contraction', 'role': 'assistant'}]