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: Pharmacology
Km value indicates: 
Select and explain. 
A. Purity of Enzyme, B. Physiological role, C. The substrate concentration at half maximal velocity, D. Aﬃnity
C: The substrate concentration at half maximal velocity
Answer: C: The substrate concentration at half maximal velocity 
Explanation: The dose rate-Cpss relationship is linear only in case of drugs eliminated by first order kinetics. For drugs (e.g. phenytoin) which follow Michaelis Menten kinetics, elimination changes from first order to zero order kinetics over the therapeutic range. Increase in their dose beyond saturation levels causes an increase in Cpss which is out of proportion to the change in dose rate.
In their case: