A 23-year-old woman complains of a sharp pain in her gums, inability to eat, foul smell from the mouth, temperature up to 38°C, general weakness. On examination, the patient is pale and her lymph nodes are enlarged. Edema, hyperemic gingival papillae, and separate ulcers covered in a fibrinous coating are observed in the patient. What additional examination needs to be performed first in this case?