What is formaldehyde primarily known as?

Formaldehyde is primarily known as an active gas in formalin, which is a solution of formaldehyde in water, typically containing methanol as a stabilizer. Formalin is widely used in laboratories and medical settings as a disinfectant and preservative. It has properties that allow it to effectively preserve biological specimens and is employed in various medical and industrial applications due to its ability to kill bacteria and fungi.

The other options do not accurately describe formaldehyde. It is not a type of protein, a skin condition, or a cystic tumor, which clarifies why the choice identifying it as an active constituent of formalin is the correct answer. Understanding formaldehyde’s primary role helps in recognizing its applications and potential health risks related to its use in products and environmental exposure.