If an error occurs during specimen preparation, at which stage can it not be rectified?

In the context of specimen preparation, fixation is a critical step that cannot be undone. This stage involves the chemical preservation of tissues, ensuring that cellular structures are stabilized and maintained for microscopic examination. Once fixation has occurred, the chemical changes that take place in the tissue cannot be reversed. It essentially 'locks in' the sample's structure at the time of fixation, preventing any further alterations or corrections to be made to the specimen in subsequent stages.

Embedding, sectioning, and mounting, while important processes in specimen preparation, allow for some degree of correction or adjustment. For example, if issues arise during embedding, the specimen can often be re-embedded properly. Similarly, sectioning allows for adjustments to be made if the initial cuts are not satisfactory, and mounting can be redone if there are problems with slides.

Therefore, fixation represents the point of no return in the specimen preparation process, as it permanently alters the tissue, making it impossible to rectify any errors that may have occurred prior to this stage.