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
[{'content': 'Microfilaments are made up of: \nSelect and explain. \nA. Actin, B. Tubulin, C. Keratin, D. None of the above', 'role': 'user'}, {'content': "Answer: A \nExplanation: Microfilaments are long solid fibers with a 4 to 6 nm diameter that are made up of actin. Although actin is most often associated with muscle contraction, it is present in all types of cells. It is the most abundant protein in mammalian cells, sometimes accounting for as much as 15% of the total protein in the cell. Ref: Ganong's Review of Medical Physiology 23rd edition, Chapter 2.", 'role': 'assistant'}]