This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Regional Anesthesia, Subject name: Anaesthesia
Adding sodium bicarbonate to local anaesthetic has all of the following effects, except: 
Select and explain. 
A. Enhances onset, B. Enhances intensity, C. Increases the duration, D. Increase the pain of injection
D: Increase the pain of injection
Answer: D: Increase the pain of injection 
Explanation: The addition of sodium bicarbonate to local anesthetics improves the speed of onset, reduces the minimum concentration of the local anesthetic required and improves the quality of block. Adding sodium bicarbonate increases the pH of the local anesthetic solution, and thus more amount of free uncharged drug is available which can increase the rate of diffusion across the nerve sheath and improve the onset of anesthesia. Subcutaneous injections of local anesthetics are painful. Alkalinization of the solution by adding sodium bicarbonate before injecting can decrease the pain.