H&E stained slides show uneven staining, with some areas well stained and others unstained. Most likely this indicates which problem?

• A. Paraffin was not completely removed
• B. Too much heat during processing
• C. Over-staining of hematoxylin
• D. Inadequate clearing

Answer: (A)

Uneven staining with patches that are well stained and others that remain unstained points to incomplete removal of paraffin before staining. After tissue is sectioned, it must be deparaffinized in xylene (or substitute) and rehydrated through graded alcohols to water. If some areas still contain wax, the staining solutions cannot penetrate those regions, so they stay pale while the rest of the tissue takes up stain normally.

Other options don’t fit this pattern as well. Excessive heat during processing can cause tissue damage or artifact but doesn’t typically create localized unstained areas. Over-staining with hematoxylin would darken nuclei everywhere rather than produce unstained patches. Inadequate clearing can affect transparency and mounting, but the specific problem of wax blocking stain penetration best explains the observed irregular staining.