**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