Which statement is true?

• A. Frozen sections are specified for demonstration of most enzymes.
• B. The epithelial surface of skin is facedown in the block.
• C. Heating the solution speeds decalcification.
• D. A chelating agent exchanges another ion for calcium.

Answer: (A)

The main idea here is that enzyme activity is easily destroyed by standard fixation, so to demonstrate enzymes histochemically, you need tissue that preserves those activities—precisely what frozen sections provide. When tissue is fixed with commonly used fixatives, enzymes are inactivated or their distribution is masked, which makes most enzyme histochemistry unreliable on paraffin-embedded specimens. Rapid freezing preserves enzyme structures and their activity long enough to apply specific histochemical stains, so frozen sections are the preferred choice for demonstrating many enzymes. This is why the statement about frozen sections being specified for demonstration of most enzymes is true.

Context helps: enzyme histochemistry relies on enzymes acting on substrates in situ, so preservation of native enzyme conformation and activity is essential. Examples include alkaline phosphatase or various dehydrogenases, where fixation would abrogate the reaction.

Regarding the other options, the orientation of skin in the block is about how you position tissue so the layer of interest appears in the initial sections; stating that the epithelial surface is facedown would misplace it from the cutting face, so it’s not the correct practice. Decalcification can be sped by heat, but this speeds the process at the expense of tissue morphology, so it’s not a universally reliable truth. And a chelating agent binds calcium by forming a complex rather than swapping an ion in an ion-exchange sense, so that wording isn’t accurate for how chelation works in decalcification.