Acid-fast staining (Ziehl–Neelsen/Kinyoun): Which reagent serves as the primary stain for acid-fast staining of mycobacteria in routine laboratory protocols?

Introduction / Context:
Acid-fast staining targets mycolic acid–rich cell walls of mycobacteria and related organisms. Recognizing the correct primary stain is crucial to avoid false negatives in tuberculosis workups and other mycobacterial infections.

Given Data / Assumptions:

• Primary stain must penetrate lipid-rich, waxy cell walls.
• Common acid-fast methods: Ziehl–Neelsen (heat) and Kinyoun (cold).
• Counterstains may vary but do not replace the primary dye.

Concept / Approach:
Carbol fuchsin (basic fuchsin with phenol) is lipid-soluble and, with heat or detergent/phenol, drives dye into mycobacterial cell walls. Acid-alcohol decolorization then removes stain from non–acid-fast cells, while acid-fast cells retain red coloration, contrasted by a blue or green counterstain for visibility.

Step-by-Step Solution:
Identify the staining system (acid-fast) and its target (mycolic acids). Recall that carbol fuchsin is the primary stain used first. Note that methylene blue is a common counterstain, not primary. Select carbol fuchsin as correct.

Verification / Alternative check:
Both Ziehl–Neelsen and Kinyoun protocols list carbol fuchsin up front, followed by acid-alcohol and counterstain.

Why Other Options Are Wrong:
Crystal violet and safranin belong to Gram stain; Giemsa is for blood smears and intracellular parasites; methylene blue is typically the counterstain in acid-fast protocols.

Common Pitfalls:
Insufficient heating or old reagents reduce sensitivity; over-decolorization can strip stain from weakly acid-fast organisms.

Final Answer:
Carbol fuchsin.