**Core Concept**
After an injury to an axon, the subsequent degeneration of axonal fibers can occur in different patterns, each with distinct characteristics. This type of degeneration is a critical aspect of neuropathology, where understanding the mechanisms and patterns of degeneration is essential for diagnosis and management.

**Why the Correct Answer is Right**
Wallerian degeneration refers to the degeneration of all axonal fibers distal to the site of injury, while the fibers proximal to the injury remain unaffected. This type of degeneration occurs due to the loss of trophic support from the cell body, leading to the breakdown of the axon and its myelin sheath. The term "Wallerian" is derived from the name of the British physiologist Augustus Waller, who first described this phenomenon in the late 19th century. The mechanism of Wallerian degeneration involves the activation of various enzymes, including calpain and caspase-3, which contribute to the breakdown of the axonal cytoskeleton and the release of neurotransmitters.

**Why Each Wrong Option is Incorrect**
**Option A:** Retrograde degeneration refers to the degeneration of the cell body and proximal axonal segments after a peripheral nerve injury, which is the opposite of Wallerian degeneration.
**Option C:** Transneuronal degeneration occurs when the axonal terminals of a neuron degenerate due to the loss of its presynaptic partner, which is a different mechanism from Wallerian degeneration.
**Option D:** This option is incorrect because Wallerian degeneration is a specific type of axonal degeneration that occurs after a nerve injury.

**Clinical Pearl / High-Yield Fact**
Wallerian degeneration is a critical concept in neuropathology, and understanding its mechanisms is essential for diagnosing and managing various neurological disorders, including peripheral neuropathies and spinal cord injuries.

**✓ Correct Answer: B. Wallerian degeneration.**