The solution responsible for staining the muscle in the image contains which of the following?

• A. Picric acid
• B. Acetic acid
• C. Eosin
• D. Hematoxylin

Answer: (A)

Muscle fibers take up dyes that color the cytoplasm, and in this image the muscle appears yellow, which points to picric acid being the staining component. Picric acid is used in certain muscle and cytoplasmic staining protocols as a counterstain or differentiator, binding to cytoplasmic proteins and giving a distinctive yellow color that contrasts with nuclei colored by hematoxylin or other elements stained by eosin. Acetic acid isn’t a dye and mainly adjusts pH, so it doesn’t produce the color seen. Eosin gives a pink/red tint to cytoplasm, not yellow, and hematoxylin stains nuclei blue/purple. Therefore, the muscle coloration comes from a solution containing picric acid.