An acid-fast bacillus with mycolic acid and a palisade arrangement is which organism?

• A. Borrelia burgdorferi
• B. Treponema pallidum
• C. Mycobacterium tuberculosis
• D. Mycobacterium leprae

Answer: (D)

The main idea here is recognizing organisms that are acid-fast due to their cell walls rich in mycolic acids, and then matching a classic tissue appearance to the organism. Mycobacteria stain acid-fast because their mycolic acid–containing cell wall retains the dye even after acid-alcohol decolorization. Among the options, the one that is a mycobacterium known to form a palisading pattern in tissue is Mycobacterium leprae. In leprosy, the bacilli can be seen arranged in a palisade or parallel stack-like pattern within granulomatous lesions. The other options are spirochetes, which are not acid-fast, or a different mycobacterial species not typically described by that palisading arrangement in standard teachings.