**Core Concept**
The Chi-square test is a statistical method used to determine whether there is a significant association between two categorical variables. It is commonly used in medical research to analyze the relationship between disease outcomes and various risk factors.

**Why the Correct Answer is Right**
The degree of freedom (df) in a Chi-square test is calculated as (r-1) * (c-1), where r represents the number of rows (or categories) and c represents the number of columns (or categories). In the context of a 2x2 contingency table, the number of rows and columns are equal, so the degree of freedom is simply (2-1) * (2-1) = 1. This is a fundamental concept in biostatistics, and understanding it is crucial for interpreting the results of a Chi-square test.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a valid formula for calculating the degree of freedom in a Chi-square test.
**Option B:** This option is incorrect because it does not take into account the number of categories in the contingency table.
**Option C:** This option is incorrect because it is not a correct formula for calculating the degree of freedom in a Chi-square test.

**Clinical Pearl / High-Yield Fact**
When performing a Chi-square test, it's essential to ensure that the expected frequency in each cell is at least 5, to avoid potential biases and ensure the validity of the results.

**Correct Answer:** . 1