Fibres least susceptible to pressure are:

**Core Concept**
Fibres least susceptible to pressure are those with the highest threshold for excitability, which is determined by their diameter and myelination status. In the peripheral nervous system, larger diameter fibres with a higher myelination ratio are more resistant to pressure-induced injury.

**Why the Correct Answer is Right**
The correct answer is large diameter, myelinated A-alpha fibres. These fibres have a relatively low excitability threshold and are responsible for proprioception, motor control, and transmission of high-frequency signals. The large diameter and myelination of A-alpha fibres reduce their susceptibility to pressure-induced damage, making them less likely to be affected by compressive forces.

**Why Each Wrong Option is Incorrect**
**Option B:** A-delta fibres, also known as fast pain fibres, are smaller in diameter and have a lower myelination ratio compared to A-alpha fibres. As a result, they are more susceptible to pressure-induced injury and are often affected by compressive forces.

**Option C:** C-fibres are small, unmyelinated fibres responsible for transmitting low-frequency signals, including pain. Due to their small diameter and lack of myelination, C-fibres are highly susceptible to pressure-induced damage and are often affected by compressive forces.

**Option D:** A-beta fibres, also known as touch and pressure fibres, are smaller in diameter and have a lower myelination ratio compared to A-alpha fibres. As a result, they are more susceptible to pressure-induced injury and are often affected by compressive forces.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that pressure-induced injury to peripheral nerves often affects smaller diameter, unmyelinated or lightly myelinated fibres, such as C-fibres and A-delta fibres, rather than larger diameter, heavily myelinated fibres like A-alpha fibres.

**Correct Answer:** A. A-alpha fibres.