A patient presents with features of ataxia. He gives a h/o a painless penile ulcer around 3 years ago that was followed by a maculopapular rash. He is staed on treatment. The MOST sensitive test to monitor him while on treatment is:

This is case of Tabes dorsalis. It is a late manifestation of syphilis that presents as symptoms and signs of demyelination of the posterior columns, dorsal roots, and dorsal root ganglia. Symptoms: Ataxic wide-based gait and foot drop Paresthesia Bladder disturbances Impotence Areflexia Loss of positional, deep-pain, and temperature sensations. The activity of neurosyphilis (symptomatic or asymptomatic) correlates best with CSF pleocytosis, and this measure provides the most sensitive index of response to treatment. Efficacy of treatment in patients with primary or secondary syphilis assessed by clinical evaluation and monitoring of the quantitative VDRL or RPR titer for a fourfold decline. Must know: Involvement of the CNS is detected by examination of CSF for pleocytosis (>5 white blood cells/L), increased protein concentration (>45 mg/dL), or VDRL reactivity. Ref: Lukeha S.A. (2012). Chapter 169. Syphilis. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.