**Question:** Investigation of choice for neurosyphilis:

A. Cerebrospinal fluid (CSF) examination
B. Serology
C. Brain imaging
D. Electroencephalogram (EEG)

**Correct Answer:** .

**Core Concept:**
Neurosyphilis is a neurological manifestation of syphilis caused by the bacterium Treponema pallidum. It can present with various neurological symptoms and signs, including cognitive decline, mood disorders, and psychiatric symptoms. Neurosyphilis can be classified into three stages: early neurosyphilis, latent neurosyphilis, and late neurosyphilis.

**Why the Correct Answer is Right:**

The correct answer, CSF examination, is essential for diagnosing neurosyphilis, as it allows for direct detection of T. pallidum and assessment of CSF parameters indicative of meningeal inflammation. A positive result for T. pallidum in the CSF confirms the diagnosis, while a negative result does not exclude it, as the disease may be in an early stage or have a non-penetrating meningeal infection.

**Why Each Wrong Option is Incorrect:**

1. Serology (Option B): Although serology can provide evidence of a previous infection with T. pallidum, it does not directly confirm the presence of the bacterium in the meninges, making it an unsuitable investigation for neurosyphilis diagnosis.
2. Brain imaging (Option C): Brain imaging can show signs of damage or inflammation in the brain but does not directly detect T. pallidum or confirm the diagnosis of neurosyphilis.
3. Electroencephalogram (EEG) (Option D): EEG is an electrophysiological test that assesses the electrical activity of the brain. It cannot confirm the presence of T. pallidum or diagnose neurosyphilis directly.

**Clinical Pearl:**

In the context of neurosyphilis, it is crucial to note that a negative result from a single test, including serology or imaging, does not exclude the diagnosis. Neurosyphilis diagnosis requires a combination of clinical suspicion, serology, and CSF examination, including microscopic examination, culture, and serology, to confirm the presence of T. pallidum and assess inflammation markers.