A 25-year-old male presents with C/O rash. There is no fever or myalgia. A slightly pruritic maculopapular rash is noted over the abdomen, trunk, palms of the hands and soles of the feet. Lymphadenopathy is also noted. Hyperophic, flat, walike lesions are noted around the anal area. Lab studies show: PCV: 40% HB: 14 g/dL WBC: 13,000/ml DLC: N50, L50 The most useful laboratory test in this patient is