This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: C.V.S., Subject name: Medicine
[{'content': 'Which of the following best describes the effect of calcium ions on the myocardium? \nSelect and explain. \nA. positively inotropic, B. negatively inotropic, C. positively chronotropic, D. negatively chronotropic', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Positively inotropic is the best description of the effect of calcium ions on the myocardium.Calcium plays a role in excitation-contraction coupling, and in possible drug effects and heart failure.', 'role': 'assistant'}]