Final answer:
The massed filaments that appear red when stained with acid-fast staining and observed under a microscope in tissues infected with Mycobacterium tuberculosis are called Acid-fast bacilli (D).
Step-by-step explanation:
The massed filaments that appear red in color due to acid-fast staining when Mycobacterium tuberculosis infected tissue is observed under a microscope are called D) Acid-fast bacilli. This staining technique is necessary because the waxy coat of mycolic acid on the bacteria’s cell wall prevents penetration by normal Gram stain reagents.
Therefore, the acid-fast stain uses carbol fuchsin as the primary stain and heat to penetrate the waxy cell wall, followed by a decolorizer. Mycobacterium tuberculosis is an acid-fast, high G+C, gram-positive bacterium. After the staining process, the bacteria retain the red color of the primary stain, while other cell types are decolorized and then counterstained with methylene blue, appearing blue.