Which of the following is a sign mounting medium evaporation as a cause?

• A. Evaporation of mounting medium
• B. Excess stain
• C. Insufficient dehydration
• D. Faulty fixative

Answer: (A)

When mounting medium evaporates, you see the effect right in the medium under the coverslip: the volume diminishes and air gaps can form, causing distortions or a closed, broken optical path as light no longer passes through a continuous medium. This direct observation—evaporation of the mounting medium itself—best identifies this cause. The other options point to staining or dehydration or fixation issues, which affect color intensity, clearing, or tissue preservation rather than the mounting medium’s evaporation.