In the Congo Red technique, nuclei appear which color because of the counterstain aluminum hematoxylin?

• A. Blue
• B. Red
• C. Pink
• D. Green

Answer: (A)

In this staining approach, amyloid is labeled with Congo Red and appears red. To clearly see cell nuclei, a blue counterstain is used—aluminum hematoxylin gives a crisp blue nuclear stain. The result is red amyloid contrasted against blue nuclei, which is why the nuclei appear blue. The other colors don’t describe the nuclei with this counterstain combination (red would reflect Congo Red–stained amyloid, pink isn’t the nuclear color here, and green isn’t produced by aluminum hematoxylin).