**Core Concept**
Fast pain is mediated by A-delta nerve fibers, which are responsible for transmitting sharp, localized pain signals to the central nervous system. A-delta fibers have a smaller diameter compared to C-fibers, which mediate slow pain.

**Why the Correct Answer is Right**
A-delta fibers transmit fast pain through the activation of nociceptors, which are specialized sensory receptors found on the surface of nerve endings. When these receptors are stimulated by a painful stimulus, they release neurotransmitters that activate A-delta fibers, leading to the transmission of fast pain signals to the spinal cord and eventually to the brain. The activation of A-delta fibers is associated with the release of substance P, a neurotransmitter that plays a key role in pain transmission.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because A-beta fibers are responsible for transmitting touch and pressure sensations, not fast pain.
**Option B:** This option is incorrect because A-gamma fibers are involved in motor control and proprioception, not pain transmission.
**Option C:** This option is incorrect because C-fibers are responsible for transmitting slow pain, not fast pain.

**Clinical Pearl / High-Yield Fact**
A-delta fibers have a conduction velocity of approximately 12-30 m/s, which is faster than C-fibers, which have a conduction velocity of approximately 2 m/s.

**Correct Answer: A. A-delta fibers.**