**Question:** Least conduction velocity is seen in

A. Myelinated nerves
B. Unmyelinated nerves
C. Nerve roots
D. Motor nerves

**Core Concept:**
The conduction velocity of a nerve impulse is directly proportional to the diameter of the axon and the density of myelin sheaths. In other words, faster conduction velocities are seen in larger myelinated axons, while slower velocities are observed in smaller, unmyelinated axons.

**Why the Correct Answer is Right:**
The correct answer, **B. Unmyelinated nerves**, exhibits the least conduction velocity because unmyelinated axons have a smaller diameter and a lower density of myelin sheaths. This leads to slower conduction velocities compared to myelinated nerves (A), nerve roots (C), and motor nerves (D).

**Why Each Wrong Option is Incorrect:**
A. Myelinated nerves: These have larger axons with a higher density of myelin sheaths, resulting in faster conduction velocities.
C. Nerve roots: Nerve roots are composed of unmyelinated axons, explaining why they exhibit slower conduction velocities.
D. Motor nerves: Motor nerves primarily contain myelinated axons, thus they have faster conduction velocities compared to unmyelinated nerves.

**Clinical Pearl / High-Yield Fact:**
Understanding nerve conduction velocity helps in differentiating between various neurological disorders, such as Guillain-Barré syndrome (a condition causing demyelination, leading to slowed conduction velocities in affected nerves) and peripheral neuropathy (a condition affecting nerve conduction velocities). This knowledge is crucial for proper diagnosis and treatment planning in clinical practice.

**Correct Answer:** B. Unmyelinated nerves