**Core Concept**
Neoplastic compressive myelopathy refers to the spinal cord compression caused by a tumor, which can lead to various neurological deficits. The most common sites of neoplastic involvement are the epidural space, particularly in the thoracic and lumbar regions.

**Why the Correct Answer is Right**
The majority of spinal neoplasms originate from the epidural space, primarily from metastatic deposits from other cancers, such as breast, lung, or prostate cancer. These tumors can spread to the epidural space through Batson's vertebral plexus, a network of veins that provides a pathway for metastatic cells. The epidural space is a fat-filled space that surrounds the dura mater, which is the outermost meningeal layer covering the spinal cord. Subdural neoplasms are less common and typically arise from the dura mater itself or from the arachnoid mater, a thinner membrane that lies between the dura mater and the pia mater.

**Why Each Wrong Option is Incorrect**
**Option B:** While the thoracic spine is a common site for spinal neoplasms, other regions such as the lumbar spine and cervical spine can also be involved. The distribution of neoplastic involvement can vary depending on the primary tumor site.
**Option C:** This statement is actually true. Neoplastic growth tends to spread in a longitudinal direction along the vertebral bodies, often crossing the disk space, rather than transversely across adjacent vertebral bodies.
**Option D:** The prognosis for neoplastic compressive myelopathy indeed worsens when motor deficits persist for more than 48 hours. Early recognition and treatment are critical to prevent irreversible neurological damage.

**Clinical Pearl / High-Yield Fact**
In patients with neoplastic compressive myelopathy, it is essential to recognize the clinical signs of spinal cord compression, including pain, weakness, sensory deficits, and bladder or bowel dysfunction, as these symptoms can progress rapidly and lead to irreversible damage if not addressed promptly.

**✓ Correct Answer: A. Most neoplasms are subdural in origin**