A 25-year-old man complains of genelal malaise, high body temperature, acute gingival bleeding, and gingival enlargement. He has a history of nosebleeds. Objectively the patient presents with systemic lymphoid hyperplasia, pallor of skin and mucosa, II-III-degree hyperplasia of the gingival mucosa, hemorrhages into the buccal mucosa, and ulcers covered with gray deposit. What examination method would be optimal for diagnosis-making in this case?