**Core Concept**
Mycobacterium tuberculosis is a bacterium responsible for causing tuberculosis (TB), a major public health concern worldwide. The discovery of M. tuberculosis is a significant milestone in the history of medicine, leading to a better understanding of the disease and its treatment.

**Why the Correct Answer is Right**
Mycobacterium tuberculosis was discovered by Robert Koch in 1882. Koch, a German physician and microbiologist, isolated the bacterium from the sputum of a TB patient and demonstrated its role in causing the disease. He used a combination of microscopy and culture techniques to identify the organism, which is now recognized as the causative agent of TB. The discovery of M. tuberculosis led to the development of Koch's postulates, a set of criteria used to establish a causal relationship between a microbe and a disease.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because M. tuberculosis was not discovered by Louis Pasteur, who is credited with the development of vaccination and germ theory.
**Option B:** This option is incorrect because Joseph Lister, a British surgeon, is known for his work in antiseptic surgery, not the discovery of M. tuberculosis.
**Option C:** This option is incorrect because Alexander Fleming, a Scottish biologist, discovered penicillin, not M. tuberculosis.

**Clinical Pearl / High-Yield Fact**
One important fact to remember is that Koch's postulates are still widely used today to establish the cause-and-effect relationship between a microbe and a disease. This concept is essential in epidemiology and clinical research.

**Correct Answer:** C. Robert Koch.