For three weeks, a 29-year-old patient has been observing a painless ulcer on the skin of the vermilion border of the lower lip The last week, the ulcer became accompanied by significant edema of the underlying and surrounding tissues. Examination shows an ulcer up to 2.5-3 cm in diameter, with. clear margins, the bottom that resembles " old lard in color, and an underlying cartilaginous infiltrate. What laboratory test must be prescribed first in this case?