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': 'Pre-capillary sphincter relaxation is caused by \nSelect and explain. \nA. Local metabolites, B. Circulating catecholamines, C. Sympathetic activity, D. Fall in capillary pressure', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Muscle fibers of the capillaries are mostly controlled by the local metabolites as it does not have much sympathetic innervation.', 'role': 'assistant'}]