## **Core Concept**
The patient's symptoms suggest a diagnosis of **Tabes Dorsalis**, a form of neurosyphilis characterized by degeneration of the posterior columns of the spinal cord and the dorsal roots. This condition results in loss of coordination (ataxia), loss of pain and temperature sensation, and autonomic dysfunction.

## **Why the Correct Answer is Right**
The correct answer, **B. Positive VDRL (Venereal Disease Research Laboratory) test**, is right because Tabes Dorsalis is a manifestation of tertiary syphilis. The VDRL test, which detects antibodies against the syphilis bacterium, can be positive in the cerebrospinal fluid (CSF) of patients with neurosyphilis, including Tabes Dorsalis. This is indicative of active infection within the central nervous system.

## **Why Each Wrong Option is Incorrect**
- **Option A:** A negative CSF VDRL test does not rule out neurosyphilis, but a positive test is supportive in the context of suggestive clinical features.
- **Option C:** The presence of **oligoclonal bands** is more indicative of multiple sclerosis or other inflammatory conditions of the central nervous system, not typically Tabes Dorsalis.
- **Option D:** **Elevated IgG index** can be seen in various conditions, including multiple sclerosis and infections like HIV, but is not specific for Tabes Dorsalis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Argyll Robertson pupils**, which are small, irregular pupils that constrict when focusing on a near object (accommodate) but do not react to light, are a classic sign of neurosyphilis, including Tabes Dorsalis. This pupillary abnormality is highly suggestive of the disease.

## **Correct Answer:** . Positive VDRL test