## **Core Concept**
The question tests the understanding of nerve conduction velocities associated with different types of nerve fibers. Nerve conduction velocity is influenced by the myelination and diameter of the nerve fibers.

## **Why the Correct Answer is Right**
The correct answer, **B.**, corresponds to large-diameter, myelinated nerve fibers, which are known for their fast conduction velocities. These fibers, often classified as Aα fibers, have diameters ranging from 12-20 μm and are involved in motor functions and proprioception. The myelination acts as an electrical insulator, and the large diameter reduces internal resistance, allowing action potentials to propagate rapidly through saltatory conduction.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option likely represents smaller, unmyelinated or thinly myelinated fibers (C fibers), which have the slowest conduction velocities (approximately 0.5-2 m/s). These fibers are involved in transmitting pain, temperature, and autonomic functions.
- **Option C:** This could represent medium-sized, myelinated fibers (Aβ), which conduct at moderate speeds (approximately 30-90 m/s) and are involved in touch and pressure sensation.
- **Option D:** This might represent small, myelinated fibers (Aδ), which conduct at slower speeds than Aα fibers but faster than C fibers, involved in transmitting pain and temperature.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that nerve conduction velocity studies (NCVS) are clinically useful in diagnosing and monitoring conditions like neuropathy. The order of conduction velocity, from fastest to slowest, generally follows: Aα > Aβ > Aδ > C fibers.

## **Correct Answer:** B. Aα fibers.