**Core Concept**
Mycobacterium tuberculosis is the causative agent of tuberculosis (TB), a chronic bacterial infection primarily affecting the lungs but also other parts of the body. The discovery of M. tuberculosis is a significant milestone in the history of microbiology.

**Why the Correct Answer is Right**
Mycobacterium tuberculosis was first isolated by Robert Koch, a German physician and microbiologist, in 1882. Koch used a combination of techniques, including microscopy and culturing, to identify the bacterium. He demonstrated the presence of the organism in sputum samples from TB patients and developed the "gold standard" criteria for establishing a causal relationship between a microbe and a disease, known as Koch's postulates.

**Why Each Wrong Option is Incorrect**
* **Option A:** Antoine Béchamp, a French biologist, is known for his work on the role of microorganisms in disease, but he did not discover M. tuberculosis. Béchamp's work focused on the role of pleomorphism, the idea that microorganisms can change their form in response to environmental factors.
* **Option B:** Louis Pasteur, a French chemist and microbiologist, made significant contributions to the field of microbiology, including the development of vaccination and pasteurization. However, he did not discover M. tuberculosis.
* **Option C:** Girolamo Fracastoro, an Italian physician, is credited with proposing the germ theory of disease in the 16th century. However, he did not discover M. tuberculosis.

**Clinical Pearl / High-Yield Fact**
The discovery of M. tuberculosis by Robert Koch marked a significant turning point in the understanding of TB and the development of diagnostic and therapeutic strategies for the disease.

**Correct Answer: C. Koch**