This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General physiology, Subject name: Physiology
[{'content': 'Which of the following is used in exocytosis \nSelect and explain. \nA. Ca2+, B. Mg2+, C. Na+, D. K+', 'role': 'user'}, {'content': "Answer: A \nExplanation: Exocytosis, in most cases, is stimulated by the entry of calcium ions into the cell; calcium ions interact with the vesicular membrane in some way that is not understood and cause its fusion with the cell membrane, followed by exocytosis--that is, opening of the membrane's outer surface and extrusion of its contents outside the cell.Ref: Guyton; 13th edition; Chapter 2; The Cell and Its Functions", 'role': 'assistant'}]