**Core Concept**
The patient's presentation, including agitation, restlessness, and neck stiffness, suggests a diagnosis of neurosyphilis, a complication of syphilis infection. This condition can occur years after the initial infection, as seen in this patient who was treated for penile ulcer 3 years ago.

**Why the Correct Answer is Right**
The diagnosis of neurosyphilis is confirmed by cerebrospinal fluid (CSF) analysis, specifically by measuring the Venereal Disease Research Laboratory (VDRL) test titer. The VDRL test detects the presence of syphilis antibodies in the CSF, which correlates with the severity of neurosyphilis. A high titer indicates more severe disease and a worse prognosis. The CSF analysis also helps differentiate neurosyphilis from other causes of neurological symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because blood tests, such as the rapid plasma reagin (RPR) test, are used to diagnose syphilis but do not provide information about the severity of neurosyphilis or its prognosis.
**Option B:** This option is incorrect because imaging studies, such as MRI or CT scans, may show abnormalities in the brain due to neurosyphilis, but they do not provide a definitive diagnosis or prognosis.
**Option C:** This option is incorrect because serum tests, such as the fluorescent treponemal antibody absorption (FTA-ABS) test, are used to confirm syphilis infection but do not provide information about the severity of neurosyphilis or its prognosis.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that a positive CSF VDRL test titer is essential for diagnosing neurosyphilis. A high titer indicates a worse prognosis and requires more aggressive treatment.

**Correct Answer:** C.