**Core Concept**
The discovery of the first pathogenic bacterium under the microscope marked a significant milestone in the field of microbiology and revolutionized our understanding of infectious diseases. Louis Pasteur and Robert Koch played pivotal roles in the development of germ theory, which posits that microorganisms are the causative agents of many diseases.

**Why the Correct Answer is Right**
The first pathogenic bacterium to be observed under the microscope was Anthrax, caused by the bacterium *Bacillus anthracis*. This bacterium was first observed by Agostino Bassi, an Italian scientist, in 1835, but it was Louis Pasteur who demonstrated the germ theory of disease using anthrax as a model organism. Pasteur's work laid the foundation for Koch's postulates, which are still used today to establish a causal relationship between a microorganism and a disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Not relevant - there is no record of a bacterium being discovered by this method before anthrax.
**Option B:** While Robert Koch made significant contributions to microbiology, he did not discover the first pathogenic bacterium under the microscope.
**Option C:** Not a bacterium - *Vibrio cholerae*, the causative agent of cholera, was not the first pathogenic bacterium to be observed.

**Clinical Pearl / High-Yield Fact**
The discovery of anthrax under the microscope marked the beginning of the germ theory of disease, which has had a profound impact on modern medicine and public health.

**Correct Answer:** C.