Which of the following is true regarding anthrax -a) M’ Fadyean reaction shows capsule b) Humans are usually resistant to infectionc) Less than 100 spores can cause pulmonary infectiond) Gram stain shows organism with bulging sporese) Sputum microscopy helps in diagnosis
**Core Concept**
Anthrax is a zoonotic disease caused by the bacterium *Bacillus anthracis*, which is a gram-positive, rod-shaped bacterium. The disease is characterized by its triphasic presentation: cutaneous, inhalational, and gastrointestinal forms. The bacterium's virulence is attributed to its ability to form a protective capsule and produce toxins.
**Why the Correct Answer is Right**
The correct answer, **Option D:** Gram stain shows organism with bulging spores, is true because *Bacillus anthracis* has a unique appearance on Gram stain due to the presence of a large, central, and bulging spore within the bacterial cell. This distinctive feature is a key diagnostic clue for anthrax. The capsule of *B. anthracis* is composed of poly-D-glutamic acid and is responsible for its virulence. The production of toxins, including lethal factor and edema factor, also contributes to the pathogenesis of anthrax.
**Why Each Wrong Option is Incorrect**
**Option A:** M'Fadyean reaction shows capsule - This statement is incorrect because the M'Fadyean reaction, also known as the Weil-Felix test, is a nonspecific agglutination test used to diagnose rickettsial infections, not anthrax.
**Option B:** Humans are usually resistant to infection - This statement is incorrect because humans are not naturally resistant to anthrax infection. The disease can affect humans through exposure to infected animals, contaminated animal products, or aerosolized spores.
**Option C:** Less than 100 spores can cause pulmonary infection - This statement is incorrect because the exact number of spores required to cause pulmonary anthrax is not well established. However, it is generally agreed that a large number of spores, often in the range of thousands, is required to cause inhalational anthrax.
**Clinical Pearl / High-Yield Fact**
Anthrax is a biothreat agent due to its high infectivity, virulence, and potential for aerosolization. Its distinctive Gram stain appearance and the presence of a central spore are key diagnostic clues.
**Correct Answer:** D. Gram stain shows organism with bulging spores