**Core Concept**
Dissociated sensory loss is a type of sensory deficit that affects specific sensory modalities while sparing others, typically involving the loss of pain and temperature sensation with preservation of light touch and proprioception. This phenomenon is often associated with lesions affecting the spinal cord, particularly in the central cord syndrome or syringomyelia.

**Why the Correct Answer is Right**
Syringomyelia is a condition characterized by the formation of a fluid-filled cavity or syrinx within the spinal cord, which can lead to central cord damage and disruption of the crossing fibers of the spinothalamic tracts responsible for transmitting pain and temperature sensation. As a result, patients with syringomyelia often experience dissociated sensory loss, with a loss of pain and temperature sensation in a "cape-like" distribution over the shoulders and chest, while light touch and proprioception remain intact.

**Why Each Wrong Option is Incorrect**
**Option B:** Vitamin B12 deficiency typically leads to a more generalized sensory neuropathy, affecting multiple sensory modalities and not typically causing dissociated sensory loss.

**Option C:** Transverse myelitis is an inflammatory condition that affects the spinal cord, causing a more diffuse and symmetrical sensory loss, rather than the specific dissociated pattern seen in syringomyelia.

**Option D:** Pellagra is a niacin deficiency that primarily affects the skin, gastrointestinal system, and nervous system, but it does not typically cause dissociated sensory loss.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that dissociated sensory loss is a classic finding in syringomyelia, and a "cape-like" distribution of sensory loss over the shoulders and chest should raise suspicion for this condition.

**✓ Correct Answer: A. Syringomyelia**