TB is diagnosed with which test?

• A. Gram stain
• B. Culture for Staphylococcus species
• C. Acid-fast bacillus stain
• D. Chest X-ray

Answer: (C)

Diagnosing tuberculosis relies on detecting the organism with a stain that highlights its distinctive cell wall. Mycobacterium tuberculosis has a waxy, lipid-rich outer layer with mycolic acids, which makes it resistant to decolorization by acid alcohol and visible when stained with an acid-fast technique. The acid-fast bacillus stain (such as Ziehl-Neelsen or auramine-rhodamine) is used on sputum or tissue to reveal acid-fast bacilli quickly, providing a rapid presumptive diagnosis that guides initial treatment and isolation.

Gram stain isn’t reliable for these bacteria because their unique cell wall doesn’t take up the typical Gram stain predictably, so it can miss the organisms or misidentify them. Chest X-ray can show patterns that suggest TB but cannot confirm infection, since similar findings occur with other diseases. Culture of the organism is definitive but takes much longer to yield results, delaying diagnosis and treatment. Therefore, the acid-fast bacillus stain is the best immediate diagnostic test among the options.