CSA (catalysed signal amplification) technique is based on which principle?

• A. Fluorescent labeling with quantum dots
• B. The ability of peroxidase enzyme to oxidize phenolic compounds to highly reactive and unstable radicals. Commonly used substrate for this is tyramine (phenol end and amine end). Amine end used to bind biotin or other molecules
• C. PCR
• D. Radioactive labeling

Answer: (B)

The idea behind Catalysed Signal Amplification is that an enzyme (usually horseradish peroxidase) generates reactive radicals from a phenolic substrate, and those radicals polymerize on the tissue to create a network that can bind many label molecules, boosting the signal. In this approach, the substrate tyramine is commonly used: the phenolic end is oxidized by the peroxidase in the presence of hydrogen peroxide to form highly reactive radicals, which then couple and form a polymeric deposit. The amine end of tyramine provides sites to bind biotin or other labeling molecules, so a single enzyme can recruit multiple labels, greatly amplifying the final signal. This is why the statement describing peroxidase-catalyzed oxidation of phenolic compounds to radicals, using tyramine to bind extra molecules, best captures the principle of CSA.

The other options describe different methods not intrinsic to CSA: fluorescent quantum dots are a labeling approach, PCR is nucleic acid amplification, and radioactive labeling involves detecting radioactivity—not the enzymatic radical polymerization central to CSA.