Type of nerve fiber most susceptible to local anaesthetic

**Core Concept**
The question is testing the susceptibility of different types of nerve fibers to local anesthetics, which is an essential concept in pharmacology and anesthesia. Local anesthetics work by blocking sodium channels, preventing the initiation and transmission of nerve impulses. The susceptibility of nerve fibers to local anesthetics varies depending on their diameter and myelination.

**Why the Correct Answer is Right**
The type of nerve fiber most susceptible to local anesthetics is the unmyelinated C fiber. This is because C fibers have a larger diameter and a higher density of sodium channels compared to myelinated fibers. As a result, local anesthetics can easily penetrate the nerve membrane and block sodium channels in C fibers, leading to a rapid onset of anesthesia. The large diameter and high sodium channel density of C fibers make them more susceptible to local anesthetics, whereas myelinated fibers are less susceptible due to their smaller diameter and lower sodium channel density.

**Why Each Wrong Option is Incorrect**
**Option A:** Myelinated A fibers are less susceptible to local anesthetics due to their smaller diameter and lower sodium channel density, which makes it harder for local anesthetics to penetrate the nerve membrane.
**Option B:** Unmyelinated B fibers are less susceptible to local anesthetics compared to C fibers, as they have a smaller diameter and lower sodium channel density.
**Option C:** Myelinated Aα fibers are less susceptible to local anesthetics due to their smaller diameter and lower sodium channel density compared to C fibers.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the susceptibility of nerve fibers to local anesthetics is a critical factor in determining the onset and duration of anesthesia. This knowledge can help anesthesiologists choose the right local anesthetic for a particular procedure and avoid potential complications.

**Correct Answer: C. Unmyelinated C fibers are most susceptible to local anesthetics.**