## **Core Concept**
Locomotor ataxia, a late manifestation of syphilis, results from the parenchymatous involvement of the spinal cord, specifically affecting the posterior columns and dorsal roots. This condition is associated with neurosyphilis, which occurs years after the initial syphilis infection. The key concept here is the involvement of the spinal cord leading to ataxia.

## **Why the Correct Answer is Right**
The correct answer, **Tabes dorsalis**, is right because it specifically refers to the degeneration of the posterior columns and dorsal roots of the spinal cord due to neurosyphilis. This degeneration leads to loss of coordination and balance (ataxia), loss of reflexes, and sensory deficits. The pathophysiology involves inflammation and degeneration of the spinal cord tissue, which disrupts the normal functioning of the posterior columns and dorsal roots.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While neurosyphilis is the broader category that includes tabes dorsalis, it is not the specific term for locomotor ataxia due to parenchymatous involvement of the spinal cord.
- **Option B:** This option is not directly related to the specific clinical manifestation described.
- **Option C:** This option does not accurately describe tabes dorsalis or the specific condition of locomotor ataxia in the context of syphilis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that tabes dorsalis typically presents with a triad of symptoms: ataxia, areflexia, and impaired sensation (particularly proprioception and vibration). A classic exam trap is to not confuse tabes dorsalis with other causes of ataxia; the specific association with syphilis and the pattern of sensory loss are critical for diagnosis.

## **Correct Answer:** D. Tabes dorsalis