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': 'Which of the following is an intracellular constituents of neuromuscular system? \nSelect and explain. \nA. Mg, B. Ca, C. Cl, D. Na', 'role': 'user'}, {'content': "Answer: A \nExplanation: The nerve and muscle fibers, like other bodily cells, maintain a fluid internal environment that is distinctly different from the external or interstitial medium. The main intracellular constituents are potassium (K), magnesium (Mg), and phosphorus (P), whereas those outside the cell are sodium (Na), calcium (Ca), and chloride (Cl). Ref: Ropper A.H., Samuels M.A. (2009). Chapter 45. Electrophysiologic and Laboratory Aids in the Diagnosis of Neuromuscular Disease. In A.H. Ropper, M.A. Samuels (Eds), Adams and Victor's Principles of Neurology, 9e.", 'role': 'assistant'}]