A decrease in section transparency can be caused by mounting medium that has become too thick.

• A. Too Thick
• B. Too Thin
• C. Too Cold
• D. Too Warm

Answer: (A)

The essential idea here is that the clarity of a stained section relies on a mounting medium that can spread evenly and form a uniform, thin layer between the tissue and the coverslip. When the mounting medium becomes too thick (too viscous), it doesn’t flow smoothly and can trap air bubbles or fail to fill all the microscopic gaps. This creates irregularities and increases the light path through areas with different refractive indices, causing light to scatter and the section to look hazy or less transparent. Temperature can influence viscosity, but the direct reason for reduced transparency is the mounting medium's excessive thickness. If the medium is warmed to reduce viscosity or replaced with fresh medium of appropriate viscosity, the transparency typically improves.