A 67-year-old man is evaluated for persistent shooting pains, lower limb ataxia, and bladder dysfunction. Physical examination demonstrates small irregular pupils that constrict with accommodation but not in response to light. A VDRL test is positive. A CT scan of the spinal cord would most likely demonstrate atrophy of which of the following structures?

**Core Concept**
The patient's symptoms and signs suggest a condition affecting the spinal cord, specifically a demyelinating disease that leads to damage of the posterior columns and autonomic pathways. The presence of small irregular pupils that constrict with accommodation but not in response to light is indicative of a lesion affecting the sympathetic and parasympathetic nerves. The positive VDRL test suggests a diagnosis of neurosyphilis.

**Why the Correct Answer is Right**
The patient's symptoms and signs are consistent with tabes dorsalis, a form of neurosyphilis that affects the posterior columns of the spinal cord. The damage to the posterior columns leads to the characteristic shooting pains, lower limb ataxia, and bladder dysfunction. The atrophy of the posterior columns would be visible on a CT scan of the spinal cord. The sympathetic and parasympathetic nerves are also affected, leading to the abnormal pupillary response. The positive VDRL test confirms the diagnosis of neurosyphilis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the lateral corticospinal tract is not primarily affected in tabes dorsalis. The damage is mainly to the posterior columns.
**Option B:** This option is incorrect because the anterior horn cells are not primarily affected in tabes dorsalis. The damage is mainly to the posterior columns.
**Option C:** This option is incorrect because the spinothalamic tract is not primarily affected in tabes dorsalis. The damage is mainly to the posterior columns.

**Clinical Pearl / High-Yield Fact**
Tabes dorsalis is a classic example of a condition that affects the posterior columns of the spinal cord, leading to the characteristic symptoms of shooting pains, lower limb ataxia, and bladder dysfunction. The positive VDRL test is a key diagnostic clue in this condition.

**Correct Answer:** C. posterior columns.

✓ Correct Answer: A. Dorsal column