A needle core biopsy of a kidney is obtained from a patient. The pathologist requests transmission electron microscopy to evaluate the specimen. Which of the following is the correct order for specimen preservation?

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For transmission electron microscopy, start with a fixative that preserves protein structure, then follow with one that stabilizes membranes and provides electron density. Glutaraldehyde in a buffered solution cross-links proteins, locking in ultrastructural details like organelles and cytoskeletal elements. After that comes osmium tetroxide, which reacts with lipids and membranes, further stabilizing them and enhancing contrast for electron microscopy. Using other sequences, such as starting with osmium tetroxide or using formalin/Bouin’s, would not preserve ultrastructure as effectively or provide the same membrane lipid stabilization and contrast required for TEM. Hence, the correct order is glutaraldehyde first, then osmium tetroxide.