What stain uses ferric chloride as a mordant?

• A. Verhoeff
• B. Gomori reticulin
• C. Aldehyde fuchsin
• D. Periodic acid-methenamine silver

Answer: (A)

Ferric chloride acts as a mordant to form a dye–metal complex that binds selectively to elastic fibers, making them appear dark, typically black. In the Verhoeff elastic stain, iron hematoxylin is used with ferric chloride to mordant the dye so it adheres strongly to elastic tissue, producing a characteristic black coloration of elastic fibers against a lighter background. The other stains rely on different mechanisms: Gomori reticulin uses silver impregnation to visualize reticular fibers; aldehyde fuchsin stains elastic fibers through a different chemical interaction involving aldehydes; and periodic acid–methenamine silver uses oxidation and silver deposition to highlight certain tissues. Thus, ferric chloride as mordant is a hallmark of the Verhoeff elastic stain.