**Core Concept**
Tuberculosis of the thoracic spine, also known as Pott's spine, is a form of extrapulmonary tuberculosis that affects the vertebral bodies and intervertebral discs, leading to vertebral collapse and spinal instability. The resulting spinal deformity can cause cord compression, which is a medical emergency.

**Why the Correct Answer is Right**
The earliest sign of cord compression in Pott's spine is usually pain, but the earliest neurological sign is often a loss of proprioception, which is the sense of the relative position of neighboring parts of the body. This is due to the compression of the posterior columns of the spinal cord, particularly the fasciculus gracilis. The posterior columns are responsible for transmitting proprioceptive information from the body to the brain. As the cord compression worsens, other neurological symptoms such as numbness, weakness, and paralysis may develop.

**Why Each Wrong Option is Incorrect**
**Option A:**
This option is incorrect because while pain is a common symptom of Pott's spine, it is not the earliest neurological sign of cord compression.

**Option B:**
This option is incorrect because while numbness is a symptom of cord compression, it is not the earliest neurological sign.

**Option C:**
This option is incorrect because while weakness is a symptom of cord compression, it is not the earliest neurological sign.

**Clinical Pearl / High-Yield Fact**
In cases of Pott's spine, the earliest neurological sign of cord compression is often a loss of proprioception, which can be detected by performing a Romberg test. This test involves asking the patient to stand with their feet together and their eyes closed; if they are unable to maintain their balance, it suggests a problem with proprioception.

**Correct Answer: A. Pain.**