Which sequence correctly lists the four primary steps of tissue processing?

• A. Fixation, Dehydration, Clearing, Infiltration
• B. Dehydration, Clearing, Infiltration, Fixation
• C. Infiltration, Embedding, Sectioning, Staining
• D. Clearing, Infiltration, Sectioning, Mounting

Answer: (A)

The main idea being tested is the correct order of tissue processing steps from preserving the tissue to preparing it for sectioning. The sequence begins with fixation, which preserves tissue structure by cross-linking proteins and stabilizing cellular components, preventing decay during processing. Next, dehydration gradually replaces water with an organic solvent (usually graded ethanol), making the tissue compatible with the embedding medium. After dehydration, clearing with a solvent such as xylene removes the alcohol and makes the tissue transparent and miscible with paraffin. Finally, infiltration with molten paraffin permeates the tissue so it can be embedded in a solid block for thin sectioning.

This order is essential because you can’t effectively clear or infiltrate tissue that still contains water, and embedding can’t happen without the tissue being infiltrated with the embedding medium. That’s why starting with dehydration or attempting infiltration or embedding before dehydration and clearing leads to poor tissue preservation and poor section quality. Sectioning and staining come after embedding, so placing them earlier would also be inappropriate.