**Core Concept**
John Snow's discovery of cholera is a landmark event in the history of epidemiology. His investigation of a cholera outbreak in London in 1854 led to the identification of a contaminated water pump as the source of the infection. This discovery marked a significant shift in understanding the role of environmental factors in the spread of disease.

**Why the Correct Answer is Right**
John Snow's work is an example of **environmental epidemiology**, which involves studying the relationship between environmental factors and the distribution of disease. By tracing the source of the cholera outbreak to a contaminated water pump, Snow demonstrated the importance of environmental factors in the spread of disease. His discovery led to the removal of the pump handle, which effectively stopped the outbreak. Snow's work laid the foundation for modern epidemiological techniques and highlighted the crucial role of environmental factors in disease transmission.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because John Snow's discovery of cholera is not an example of **genetic epidemiology**, which involves studying the relationship between genetic factors and the distribution of disease.
* **Option B:** This option is incorrect because John Snow's work on cholera is not an example of **pharmacological epidemiology**, which involves studying the relationship between medications and the distribution of disease.
* **Option C:** This option is incorrect because John Snow's discovery of cholera is not an example of **molecular epidemiology**, which involves studying the molecular mechanisms of disease transmission.

**Clinical Pearl / High-Yield Fact**
John Snow's discovery of cholera highlights the importance of environmental factors in disease transmission and demonstrates the value of epidemiological investigation in identifying the source of outbreaks. This approach has been applied to numerous infectious disease outbreaks throughout history and remains a crucial tool in modern public health practice.

**Correct Answer: D. Environmental epidemiology.**