## **Core Concept**
The secondary attack rate is a measure used in epidemiology to quantify the spread of a disease within a defined group, such as a household. It is calculated as the number of new cases among the contacts of the primary case(s) divided by the number of exposed contacts. The formula for secondary attack rate is: (Number of new cases among contacts / Number of exposed contacts) * 100.

## **Why the Correct Answer is Right**
Given that there is 1 primary case and 2 secondary cases among 8 contacts (since one of the 9 individuals was the initial infected), the secondary attack rate can be calculated. The number of exposed contacts is 8 (excluding the primary case). The correct calculation is: (2 / 8) * 100 = 25%. This matches option **C. 25%**, making it the correct answer.

## **Why Each Wrong Option is Incorrect**
- **Option A: 11%** - This would imply (1/9) * 100, which incorrectly includes the primary case in the calculation and does not account for the timing of infections.
- **Option B: 20%** - This would imply (2/10) * 100, which incorrectly includes an additional person in the denominator, possibly assuming a different number of contacts or misinterpreting the question.
- **Option D: 30%** - This does not match any straightforward calculation based on the provided scenario.

## **Clinical Pearl / High-Yield Fact**
The secondary attack rate is particularly useful for diseases that are highly contagious and spread through close contact, such as influenza or COVID-19. A higher secondary attack rate indicates a more contagious disease or a more infectious agent.

## **Correct Answer: C. 25%.**