During induction of local anesthesia with lidocaine, nerve fibers are blocked in a characteristic sequence. Which fiber type loses function earliest?

• A Large myelinated A-alpha motor fibers
• B Small unmyelinated C pain fibers
• C Small myelinated B preganglionic sympathetic fibers ✓
• D A-beta touch fibers

Explanation

Sensitivity to local anesthetics correlates inversely with fiber diameter and directly with firing frequency, because lidocaine binds preferentially to open and inactivated sodium channels. The classic clinical order of blockade is C fibers first, then B, then A-delta, then A-beta and A-gamma, with large A-alpha motor fibers last. This explains why sympathetic and pain functions are abolished before motor power, contradicting option A.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.