This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Pharmacokinetics, Subject name: Pharmacology
Which of the following factors is affected by the bioavailability of a drug? 
Select and explain. 
A. Half life of the drug, B. Volume of distribution, C. Dose of the drug, D. pKa value
C: Dose of the drug
Answer: C: Dose of the drug 
Explanation: * Bioavailability - Fraction of dose reaching the systemic circulation. - Determines the dose High bioavailability - low dose Low bioavailability - high dose Factors affecting bioavailability 1. Absorption More absorption , more bioavailability 2. First pass metabolism More first pass metabolism, less bioavailability Half Life Vd= amount of drug/ plasma concentration * Both factors (Half life and Vd) are not related to bioavailability * pKa is the pH of the medium at which 50% of the drug can cross the membrane and 50% cannot cross the membrane.