A 25-year-old male student presents with the chief complaint of rash. He denies headache, fever, or myalgia. A slightly pruritic maculopapular rash is noted over the abdomen, trunk, palms of the hands, and soles of the feet. Inguinal, occipital, and cervical lymphadenopathy is also noted. Hypertrophic, flat, wart-like lesions are noted around the anal area. Laboratory studies show the following:Hct: 40%Hgb: 14 g/dLWBC: 13,000/mLDiff: 50% segmented neutrophils, 50% lymphocytesWhich of the following is the most useful laboratory test in this patient?