Which fixative combination includes Formaldehyde, NaCl, and DI water and is isotonic but may produce formalin pigment?

• A. 10% Formalin Saline
• B. Calcium Formalin
• C. Formalin Ammonium Bromide
• D. Acetate Formalin

Answer: (A)

Isotonic fixation relies on adding salt to match the tissue’s osmolarity so cells don’t shrink or swell during fixation. A saline-based formalin solution does exactly that: it uses formaldehyde for tissue preservation while NaCl provides the isotonic environment, with DI water simply diluting to the desired 10% formalin saline. Because this fixative is unbuffered, its pH can drift, and formaldehyde can form polymerization products that appear as a brown to black granular pigment known as formalin pigment in the tissue. Other options introduce different ions or buffering that change the osmolarity or pH profile, so they don’t fit the description of including Formaldehyde, NaCl, and DI water while remaining isotonic.