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: Physiology
True regarding excitation contraction coupling in smooth muscles is 
Select and explain. 
A. 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
D: Presence of cellular calcium is essential to cause muscle contraction
Answer: D: Presence of cellular calcium is essential to cause muscle contraction 
Explanation: Troponin is needed for skeletal muscle contraction, not smooth muscle. Sustained contraction occurs due to ‘latch’. 
Myosin needs to be phosphorylated rather than actin for smooth muscle contraction.