___ acid (staining agent used in electron microscopy)

OSMIC

This heavy metal compound, derived from osmium, is known for its sharp, pungent odor and is often used to fix and stain lipids in biological samples.

This clue is looking for a type of acid commonly used in electron microscopy to stain biological samples. "Osmic acid" is another name for osmium tetroxide, a chemical that binds to lipids and enhances contrast in microscopic images. The blank before "acid" signals you’re filling in the first part of the compound name, which is simply "osmic." No wordplay here—just a direct reference to a lab staple.