## **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 family members or close contacts, over a specific period. It is calculated as the number of new cases among contacts divided by the number of exposed contacts. For measles, which is highly contagious, the secondary attack rate can be particularly high, especially among susceptible contacts.

## **Why the Correct Answer is Right**
The secondary attack rate for measles in this scenario can be calculated as the number of new cases (6) divided by the total number of exposed contacts (10), which equals 60%. This measure indicates that 60% of the exposed contacts developed measles. The formula for the secondary attack rate is: (Number of new cases among contacts / Number of exposed contacts) * 100. Applying this formula: (6 / 10) * 100 = 60%.

## **Why Each Wrong Option is Incorrect**
- **Option A:** If the calculation or given data suggested a rate of 40%, it would not match our calculated rate of 60%, making it incorrect.
- **Option B:** Similarly, if another option suggested a rate different from 60%, based on incorrect calculation or assumption, it would be wrong.
- **Option D:** This option would also be incorrect for the same reason as options A and B, not matching the calculated secondary attack rate of 60%.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that measles has a very high secondary attack rate, typically cited around 90% for susceptible contacts, making it one of the most contagious diseases known. However, this can vary based on factors like vaccination status of the population and the presence of any immunity. The scenario provided illustrates a high but not exceptionally high rate, possibly due to partial immunity or other protective factors within the group.

## **Correct Answer: B. 60%**