Question

A 35-year-old female was watching TV for long hours with her hands under her head. She complains of a tingling sensation over her arm. Which type of nerve fibers is most likely to be affected?

• A. A fibers
• B. B fibers
• C. C fibers
• D. Sympathetic nerve fibers

Hint:

Pressure block hits the large myelinated fibers first; local anaesthetic hits the small ones first.

Answer 1

The Correct Option is A

Remember the rule of opposites for nerve fiber vulnerability. Pressure preferentially blocks large myelinated A fibers, while local anaesthetics preferentially block small unmyelinated C fibers. The clinical story here, a compressed arm going numb and tingly after prolonged mechanical pressure, points straight to a pressure block of A fibers, so option (a) is correct.
The reason is size and myelination. Large A fibers have the highest metabolic demand at their nodes and are physically deformed most by compression, so conduction fails in them first when blood supply and pressure are unfavourable. The small unmyelinated C fibers keep conducting longest under pressure, which is why pain and temperature sometimes persist even when touch and position sense fade. B and sympathetic fibers are not the answer for a purely mechanical compression scenario. Choose A fibers.
Ref: Arvind Arora Review Book of Physiology.