A person working in an abattoir presented with pustule on hand which turned into ulcer. Which of the following will best help in diagnosis?

Cutaneous anthrax generally occurs on exposed surfaces of the arms or hands. A pruritic papule develops 1-7 days after entry of the organisms and rapidly changes into a necrotic ulcer. The lesion is called a malignant pustule. The anthrax bacillus is Gram positive and nonacid fast. The spores do not stain by ordinary methods. When blood films containing anthrax bacilli are stained with polychrome methylene blue for a few seconds and are examined under microscope, an amorphous purplish material is noticed around the bacilli. This represents the capsular material and is characteristic of the anthrax bacillus. This is called "McFadyean's reaction" and is employed for the presumptive diagnosis of anthrax. Ref: Ananthanarayan and Paniker's Textbook of Microbiology By Ananthanarayan And Paniker, 2006, Page 242.