This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pharmacodynamics, Subject name: Pharmacology
True about pKa is:- 
Select and explain. 
A. pH at which ionized fraction of drug equals to unionized fraction, B. pH at which ionized fraction of drug is more than unionized fraction, C. pH at which ionized fraction of drug is less than unionized fraction, D. pH at which ionized fraction of drug is twice unionized fraction
A: pH at which ionized fraction of drug equals to unionized fraction
Answer: A: pH at which ionized fraction of drug equals to unionized fraction 
Explanation: * pKa is the pH at which ionized fraction(50%) of drug equals to unionized fraction(50%). * i.e. 50% is lipid soluble and 50% is water soluble. * It is the pH at which 50% of drug can cross the membrane and 50% of drug cannot cross the membrane.