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: Dental
Fluid flow in or out of the dentinal tubules is proportional to the tubule diameter by 
Select and explain. 
A. Third power of the diameter, B. Sixth power of the diameter, C. Square of diameter, D. Fourth power of diameter
D: Fourth power of diameter
Answer: D: Fourth power of diameter 
Explanation: fluid flow in dentin tubules follows Poiseuille’s law as all tubular structures:
Q=Pr4/8nl
Q= blood flow
P= pressure difference
N= velocity
R= radius of vessel
L= length of vessel