Which chromogens are listed for HRP substrate?

• A. DAB, ACE
• B. BCIP/NBT
• C. Fast Red TR salt
• D. AEC

Answer: (D)

When horseradish peroxidase (HRP) is used in immunohistochemistry, a chromogen is required to produce a visible colored deposit where the target antigen is located. Aminoethyl carbazole (AEC) is a red chromogen that is specifically listed for HRP substrates in many protocols. It reacts with HRP in the presence of hydrogen peroxide to yield a distinct red/orange precipitate, making positive staining easy to see under a light microscope.

In this context, the listed HRP substrate chromogen is AEC, which is why it’s the best answer. The other options correspond to substrates used with different enzymes or contexts: BCIP/NBT is for alkaline phosphatase and yields a blue product; Fast Red TR salt can be used with HRP in some setups but isn’t the chromogen listed here; DAB is a very common HRP chromogen as well, but in this particular question’s listed options it isn’t identified as the HRP substrate chromogen.