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
Precapillary sphincter relaxation is caused by 
Select and explain. 
A. Sympathetic activity, B. Circulating catecholamines, C. Local metabolites, D. Fall in capillary pressure
C: Local metabolites
Answer: C: Local metabolites 
Explanation: There is no autonomic supply to precapillary sphincters but control of these sphincters is done by oxygen concentration in tissue and local metabolites.