This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Muscle, Subject name: Physiology
[{'content': 'Which of the following is NOT true for a smooth muscle? \nSelect and explain. \nA. no Z-discs present, B. Actin : myosin ratio = 2 : 1, C. The contraction is "thick filament-driven", D. "Latch bridge" observed.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: There are no Z-lines in smooth muscle. Z-lines are replaced with dense bodies. Thin : thick filament ratio is 15 : 1. When Ca2+ combines with calmodulin, there is activation of the enzyme myosin light chain kinase (MLCK), and the actin-myosin interaction can begin. Myosin light chains in the thick filament thus regulates the contraction. "Latch-bridge" phenomenon causes smooth muscle to develop sustained tension for a lesser energy consumption.', 'role': 'assistant'}]