## Core Concept
McKeown's theory explains the decline in mortality rates from infectious diseases, including tuberculosis, in the 20th century. The theory primarily focuses on the role of factors such as improved nutrition, better living standards, and changes in disease transmission dynamics.

## Why the Correct Answer is Right
The correct answer, **improved nutrition**, is supported by McKeown's findings that the reduction in mortality from tuberculosis and other infectious diseases was largely due to improvements in living standards, particularly better nutrition. Improved nutrition enhances the body's immune response, making individuals less susceptible to infections and better equipped to fight off diseases like tuberculosis. This factor played a significant role in reducing mortality rates before the widespread use of specific treatments like antibiotics.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, incorrect options might include factors like "increased use of antibiotics," "changes in disease transmission," or "improved sanitation." While these factors did contribute to the decline in infectious diseases, McKeown specifically emphasized the role of improved nutrition and living standards.
- **Option B:** Similarly, without the content, we can infer that if an option suggests "public health measures" or "vaccination," these are important but were not McKeown's primary focus.
- **Option D:** If an option like "increased exposure to TB" were provided, it would be incorrect because increased exposure would likely increase, not decrease, mortality rates.

## Clinical Pearl / High-Yield Fact
A key point to remember is that McKeown's theory underscores the importance of **non-medical factors** in the decline of infectious diseases. This challenges the common assumption that medical interventions alone are responsible for improvements in public health. Specifically, for tuberculosis, better nutrition and living conditions helped reduce susceptibility and mortality, highlighting the interplay between socioeconomic factors and disease outcomes.

## Correct Answer: C. improved nutrition