This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Other topics and Adverse effects, Subject name: Pharmacology
Epinephrine added to a solution of lignocaine for a peripheral nerve block will: 
Select and explain. 
A. Increase risk of convulsions, B. Increase the duration of action of the local anesthetic, C. Both (a) and (b), D. None of these
B: Increase the duration of action of the local anesthetic
Answer: B: Increase the duration of action of the local anesthetic 
Explanation: (Ref: K DT 6/e p354) Adrenaline and felypressin are the vasoconstrictors that are added to LA solution. By causing vasoconstriction, these drugs - decrease the systemic absorption resulting in less CNS adverse effects (decreased chances of seizures). Prolong the stay of drug at the site of action resulting in the increase in duration of action of LA.