This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Anaesthesia, Subject name: Pharmacology
[{'content': 'An agent added to local anesthetics to speed the onset of action is: \nSelect and explain. \nA. Methylparapben, B. Bicarbonate, C. Fentanyl, D. Adrenaline', 'role': 'user'}, {'content': 'Answer: B \nExplanation: * LAs are weak bases. These require penetration inside the neuron for their action. For entry in the neuron, LAs have to cross the neuronal membrane. * Unionized drugs (lipid soluble) can easily cross the membrane, therefore addition of NaHCO3 in the local anaesthetic solution (weak bases are un-ionized in the alkaline medium) makes them rapid acting. * Adrenaline increases the duration of action by causing vasoconstriction. * Methylparapben is the preservative added in LA solution.', 'role': 'assistant'}]